Depends on your resolution. For that PC though, I say if you're gaming at 1680x1050 or 1080p then a GTS 450 or Radeon 5770. A GTX 460 or 6850 is probably overkill even at 1080p as long as you use that old CPU. If a game is running slow it will more likely be the CPU hold it back. If your resolution is 1440x900 or lower, then go with a Radeon 5670.