I think it's also very important to the resolution you want to game on.
1680x1050 = Radeon 4850 / GTS250 and equivalents.
1920x1080 = GTX460 / Radeon 6850 and equivalents.
The higher resolution you go, the better the videocards you need to have.
Generally, the processor will dictate your FPS in a high stress situation, as well the enabling the extra graphical options. While the video card dictates the fastest FPS you can get on a game and the resolution you can set without slowing the game down.