Memory Leaks have been a problem for GalCiv2 since its release, and I applaud the developers for their tireless and dedicated efforts in attempting to resolve these leaks.
That said, the leaks are still an ever-present and annoying issue inherent to the game. Typically they don't manifest themselves until a saved game starts to reach a certain maturity level (say, once everyone starts nearing the end of the tech tree, and/or everyone has a ton of ships flying around). The biggest challenges to the RAM usage of the game seem to be:
1) The ship config files
2) The number of ships in play at any given time
3) The number of ship designs in play at any given time
4) Use of the Shipyard to design ships
5) Battles between ships
6) Planetary invasions
7) Game saves
8) Galaxy size
These instances burn a slow, but steady hole in system memory allocation -- eventually causing a game to grow more and more unstable as it progresses. In a typical game, for instance, I'll experience no problems at all until I reach a certain tech level, and the game has become particularly deep and involved. By then, the game will begin to crash nearly every time I design a new ship, or every handful of turns. Usually the game will freeze, but occasionally it will crash-to-desktop with an "out of memory" error message.
Just to give you a sense of my system setup: I'm running a 2GHz Core Duo laptop with 2GB RAM and a 4GB paging file. I have a fairly recent video card with 256MB of memory. I defragment my hard drive regularly, I run only GalCiv2 when I'm playing the game, and usually I quit / turn off my machine after an hour or two of play. Also, I clear out my ship config files and saved game files after every game runs its course. Basically, there is nothing about my system or my gaming habits that should cause this crash situation to happen.