This all depends how would you like to play the games.
Just want them to run or hardcore gaming?

Geforce 5700 Fx and 512MB RAM will run all the games out yet but if you want hardcore gaming and want them to run at max at 1600x1200 with reasonable framerates then go for 2GB RAM, your board does'nt support DDRII so buy DDR3200 400MHz and go far a Geforce 6800 GT/Ultra. Don't even think of purchasing above then this one.
Why i am saying this? Most of you will say that just because i have Geforce 6800 GT.
No the reason is not the same. If you buy a 7900GTX for $500+ then you are just wasting the money, because this card supports DirectX 9.0c and the windows VISTA retail will ship with DirectX 10 and NVIDIA & ATi have promised that DX10 card will be available at the end of 2006, so i better suggest you to wait for them to come.Geforce 6800GT/ULTRA will run all games at max with full antialiasing with high framerates.