what shall i do to my pc at mo iv got windows xp, pentium d 3ghz (single core) 1 gig ram Nvidia7600gt. im thinking of going for 2gig ram but i dont know got about £300 to spend and i might get a 19-20inch screen but my screen at mo is good so i might not go for that. what do u guys think. Maby go for double core and extra gig of ram i dont know. Sins runs fine now but when i get to like 100 planets 10 players and there is loads going on like when i have 30 planets then it starts stuttering is that the mem or the single cores fault?
First off before you start picking out any parts, which graphics interface is your current 7600GT on? Is it PCI-E or AGP?
After you find that out, then you can look at what options there are for a better card. If it's AGP, then the best nvidia option is the 7900GS from XFX. If it's PCI-E, then the best nvidia card is the 8900GT (the 8800GTX performs just as well and cheaper).
If you're using DDR2 RAM, you shouldn't need to spend anywhere near 300 Euros. Around 150-180 Euros should cover 2 1GB sticks of DDR2 RAM. I would just upgrade RAM to 2 GBs. It offers the performance needed when you reach 100 planets.
You mentioned you have a Pentium D (3Ghz). That is a dual-core CPU already. Intel doesn't make single-core Pentium Ds (the D means Dual-core). And if it is indeed 3Ghz, then you most likely have a socket LGA 755 and prolly using the Pentium D 945. It performs about as well as a Core 2 4320.
As for a processor upgrade, I'd check to see if your current BIOS version on your HP machine supports Core 2 Duos and which ones it supports. Just b/c the board is socket 755 doesn't mean the BIOS will recognize a Core 2 Duo.