its AGP, probably a 6800GT, look for the pre overclocked ones.
If you like ATI the X800XT PE would be the best one you could get but most people on this forum like Nvidia so i may get flamed about this, haha.
Personally i think 6800gt is good for you. I have a 6600gt and it is very good value for money, but there is also that new 6800 that just come out...