Well, it's hard to say exactly what causes it. I am playing on a Huge map with 7 opponents. After playing to the point where all planets are taken, of course wars break out. Sometimes, when I get invaded, the invading ship has 0 soldiers on it, and it just keeps invading the same planet over and over again. Eventually the game runs out of memory and crashes.
As you can see by the debug log, I do not have a small amount of RAM in this machine

Debug Message: Version v1.3 beta 1 last updated on: Thu Jul 27 18:22:18 2006
Debug Message: *********DXDiag info follows.*********
System Info
Time: 7/29/2006, 14:15:12
DirectX Version: DirectX 9.0c (4.09.0000.0904)
Machine Name: P4
Operating System: Windows 2000 Professional (5.0, Build 2195) Service Pack 4
Languages: English (Regional Setting: English)
System Manufacturer: INTEL_
System Model: D865PERL
BIOS: BIOS Date: 04/02/04 13:33:00 Ver: 08.00.10
Processor: Intel(R) Pentium(R) 4 CPU 3.00GHz (2 CPUs), ~3.0GHz
Physical Memory: 2046MB RAM
Page File: 249MB used, 5738MB available
Windows Dir: C:\WINNT
Display Info
Display device 1:
Device Name: \\.\DISPLAY1
Card Name: ALL-IN-WONDER X800 Series
Manufacturer: ATI Technologies Inc.
Chip Type: RADEON X800 XT AGP (0x4A4B)
DAC Type: Internal DAC(400MHz)
Key Device: Enum\PCI\VEN_1002&DEV_4A4B&SUBSYS_44221002&REV_00
Display Memory: 256.0 MB
Current Display Mode: 1792 x 1344 (32 bit) (60Hz)
Monitor Name: Plug and Play Monitor
Monitor Max Resolution: 1600,1200
Driver Name: ati2dvag.dll
Driver Version: 6.14.0010.6599
Driver Date: 1/24/2006 20:52:48
DirectX Acceleration Enabled: Yes
Sound info
Sound device 1:
Description: SoundMAX Digital Audio
Default Sound Playback: Yes
Default Voice Playback: Yes
Hardware ID: PCI\VEN_8086&DEV_24D5&SUBSYS_A0008086&REV_02
Manufacturer ID: 1
Product ID: 100
Driver Name: smwdm.sys
Provider: Analog Devices
Driver Version: 5.12.0001.5240
Driver Date: 9/1/2004 13:18:40
Type: WDM
Other Drivers:
Acceleration Level: 3
Cap Flags: 0
Min/Max Sample Rate: 0, 0
Debug Message: *********END dxdiag info.*********
Debug Message: Entering WinMain
Debug Message: ******* Starting Game Shell *******
Debug Message: Preparing to change display settings.
Debug Message: Using multilsample setting: D3DMULTISAMPLE_NONE
Debug Message: Game Resolution: 1792 x 1344
Debug Message: Running in windowed mode.
Debug Message: HAL (hw vp): ALL-IN-WONDER X800 Series
Debug Message: D3DPRESENT_INTERVAL_IMMEDIATE
Debug Message: Refresh rate: 0
Debug Message: Bit Depth: 32 bit.
Debug Message: *********Checking D3DDevice Caps***********************
Debug Message: A full-color cursor is supported in hardware at high resolution modes.
Debug Message: Border size reported as 6 by 25
Debug Message: RGB in format 6:6:6, Masks reported as ff0000:ff00:ff
Debug Message: Now windowed.
Debug Message: Shell initialization complete
Debug Message: ***** Initializing Game *****
Debug Message: Timer Frequency is 0 b25bca40
Debug Message: Initializing sound card.
Debug Message: initailizing bink
Debug Message: Create setup screens.
Debug Message: Clear galaxy settings.
Debug Message: Clear influence.
Debug Message: Clear up laws.
Debug Message: Clear first time events.
Debug Message: Clear wonders and trade goods.
Debug Message: Clear sector mapper.
Debug Message: * Collectors Edition *
Debug Message: Set mouse.
Debug Message: Set update timer.
Debug Message: Clear data definitions.
Debug Message: Reading Star Names
Debug Message: Reading Star Names... Done
Debug Message: No files found matching mask C:\Documents and Settings\username_removed\My Documents\My Games\GalCiv2\Logos\*.sav
Debug Message: No files found matching mask C:\Documents and Settings\username_removed\My Documents\My Games\GalCiv2\Metaverse\*.sav
Debug Message: No files found matching mask C:\Documents and Settings\username_removed\My Documents\My Games\GalCiv2\Mods\*.sav
Debug Message: No files found matching mask C:\Documents and Settings\username_removed\My Documents\My Games\GalCiv2\Portraits\*.sav
Debug Message: No files found matching mask C:\Documents and Settings\username_removed\My Documents\My Games\GalCiv2\ships\*.sav
Debug Message: No files found matching mask C:\Documents and Settings\username_removed\My Documents\My Games\GalCiv2\temp\*.sav
Debug Message: Killing threads.
Debug Message: Kiling civs.
Debug Message: Killing ships.
Debug Message: Killing rallypoints.
Debug Message: Killing misc objects.
Debug Message: Killing stars.
Debug Message: Killing anomalies.
Debug Message: Killing space resources.
Debug Message: Killing colonies and planets.
Debug Message: killing scenario.
Debug Message: Clearing Turn Event Log
Debug Message: Clear galaxy settings.
Debug Message: Clear influence.
Debug Message: Clear up laws.
Debug Message: Clear first time events.
Debug Message: Clear wonders and trade goods.
Debug Message: Clear sector mapper.
Debug Message: Loading Custom Map
Debug Message: Loading Data Defs
Debug Message: Clear data definitions.
Debug Message: Loading TechDefs
Debug Message: Max num techs: 248
Debug Message: Loading ImprovementDB
Debug Message: Loading Component Defs
Debug Message: Loading ShipTypes
Debug Message: Loading ShipConfigs
Debug Message: Loading AnomalyTypes
Debug Message: Loading RandomEvents
Debug Message: Loading UPIssues
Debug Message: Loading ResourceTypes
Debug Message: Loading InvasionTactics
Debug Message: Loading SBmodules
Debug Message: Loading PoliticalParties
Debug Message: Loading Planet Types
Debug Message: Loading Star Types
Debug Message: Loading Game
Debug Message: Loading: game mode is metaverse.
Debug Message: Loading Galaxy
Debug Message: Sectors across: 12 x 12
Debug Message: Loading FOW Map
Debug Message: Loading ZOC Map
Debug Message: Loading Planets
Debug Message: Loading Stars
Debug Message: Loading Anomalies
Debug Message: Loading Resources
Debug Message: Loading Civs
Debug Message: Loading Colonies
Debug Message: Loading RallyPoints
Debug Message: Loading Ships
Debug Message: Loading Fleets
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Can't find ship type Fleet in definitions
Debug Message: Loading Turn Event Log
Debug Message: Loading ENDGALAXYDATA
Debug Message: Updating ZOC.
Debug Message: UIBeginGame called with g_ulLocalPlayerID = 4
Debug Message: UIBeginGameHelper: New Metaverse Game Started
Debug Message: UIBeginGameHelper: Mods are Disabled
Debug Message: BuildShip: Cannot build ship type 3-Heavy Fighter M4-9 at Acelloria I - No open slots
Debug Message: StackedShipsWnd: Cannot find OBJID Stacked_Logistics_Value
Debug Message: StackedShipsWnd: Cannot find OBJID stacked_Fleet_More_Button
Debug Message: Save Game: About to save
Debug Message: Saving game C:\Documents and Settings\username_removed\My Documents\My Games\GalCiv2\QuickSave.sav
Debug Message: Num Sectors Across: 12
Debug Message: Saving: game mode is metaverse.
ASSERT ERROR in file .\Source\memman.cpp at line 1384.
ASSERT ERROR in file .\Source\memman.cpp at line 978.
Debug Message: Memory overflow detected in memory system UIGame