The GeForce 6600/6600GT or Radeon X700/X700Pro are you're best bets, although the 6800's are getting around the $130 online, and that would certainly be worth the move. Having said that, with a decent CPU and 512MB-1GB of RAM, you can play stuff like Doom 3 with pretty good detail. I can't say you'll do quite as well with FEAR or Oblivion, but I do know that Doom 3-based games will run nicely with the 6600 or X700.