I guess I dont understan. Why would 3 GTX 480's (3 GPU's) be better than 2 HD 5970's (4 GPU's). Am i missing something?
(1) gtx 480 beats (1) radeon 5870 by a decent margin, which is the GPU used in the 5970, and quad crossfire does not scale well.
However, you also have to consider power consumption + heat. a 5970 beats gtx 480 in that category.