How much CPU performance do you really need to adequately utilize a current-generation AMD graphics card? Since this is a CrossFire test, the CPU must be powerful enough to keep both Radeon HD 5850 GPUs maxed out simultaneously.
In order to measure how CrossFire performance scales with CPU performance, we overclocked the Core i5 CPU from 2.66 GHz to 3.99 GHz in eight steps. Past experience with CrossFire would suggest that a bit of extra CPU performance surely can't hurt if you want to squeeze out those extra frames per second. However, modern high-resolution gaming with lots of anti-aliasing puts most of the load on the graphics cards.
Increasing CPU performance hardly seems to have any effect at all when using the newest 3D engines. Only the slightly older DirectX 9 Source engine (Half Life 2, Left 4 Dead, Left 4 Dead 2) shows meaningful differences.