As always, if the reason to ask about performance is an Open GL graphics program, an nVidia card is ahead. If it's for any particular game, and you know the game uses Direct3D, that performance lead is lost. In either NWN2 or Oblivion, the X1600 Pro handles those games better than a 6600 GT. ATI uses an architecture that deals with the Dx9 shaders better than nVidia, up to the 8800 design.