It will depend on the game, obviously. If the game uses PhysX and you have the hardware physics enabled a cheap Geforce will give you a lot better performance. Probably you wouldn't be able to enable hardware physics without a Geforce or Ageia ppu, there's the possibility that some few games wouldn't even work, so in reality it's either more frames with no enhanced gameplay, or less frames but outstanding physics. Choose what you prefer. The whole point of this is to have a lot better physics than what an entire (4 cores) Overclocked Quad can handle. Even the 8400 GS has probably more number crunching power than the fastest quad.