Sounds like your video card is not quite up to snuff... I've found the bottom end to be around the geforce 9000 series. It plays on my old 1.7 ghz machine with the 9800.
I suggest the newer nvidia midrange cards. I bought a gt220 i think it was, not too long ago, only about $100 and it made all the difference. The game should scream on your machine if it's 3 ghz or more, you just need to upgrade the video which you should be able to do for less than $100. It will make other games fly too...
Good luck, post back if you have any more questions.
Gambit Realm development team