You can get a new graphics card, here are some new fine choices.
6800GS-$200
7800GT-$300
7800GTX-$400-780 ( 256, 512mb)
You might want to get some new ram also, I read that you have all types of ram in your system, that's not really good, if I could recommend a 1 gig kit it would be the 2x512 mb OCZ gold series.