X-bit Labs compared the performance of the NVIDIA GeForce GTX 285 and GeForce GTX 295 graphics cards on a Intel Core 2 Quad QX9650 and Core i7 920 system. You can check it out over here, below is a small snip from the conclusion:
What did we learn from today’s test session? Against my expectations, a fast platform with a Core i7 processor will come in handy for a GeForce GTX 295 graphics card in order to increase gaming performance. On the other hand, you should be aware that there are games and modes in which speed is limited by the graphics subsystem only. Particularly, these are Devil May Cry 4, S.T.A.L.K.E.R.: Clear Sky, Crysis, Far Cry 2 and the synthetic 3DMark Vantage. Considering that such cards are bought with the purpose of playing games at high settings and high resolutions, the role of the CPU becomes insignificant. As for the GeForce GTX 285, it is about 40% slower than the dual-processor GeForce GTX 295 and is a less CPU-dependent graphics card.