Your system should run just fine with windows and 3 gigs.

I am running about the same setup you are without a problem Cities don't give me any trouble.
The only difference is I have 4gigs but being on windows xp my comp only sees the max of 3.45 gigs.

I think some of your sticks are bad or loose. Check that first before you go upgrading anything. Make sure you have the right ram in it also.

Check your video settings. I had to force my computer to see the last gig. I don't remember how we did it either. You might also try taking out 4 gigs. They are pointless to have in your system as it sits right now anyway.

Edit.. I noticed your asking about upgrading your video card. I was 5 boxing off of a 7600 card again with little trouble. Your card is fine. I would stick with something is wrong with your ram. IMO signs are there with it only seeing the 2gigs. It should see 3 at least...