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...
Core 2 Duo E6400, DFI Infinity 975X/G, 2x 512mb DDR2 667mhz, Albatron 7900gt, WD 200gb SATA, Samsung DVD-RW, Silverstone ST-50EF 500w PSU.