^+1, the HD 5970 is two HD 5870s underclocked to HD 5850 speeds, and therefore it performs like an HD 5850 in CrossFireX. However, dual GPU scaling is OK, while quad CFX is quite buggy and can actually provide lower rates.
The GTX 480 is beaten by the HD 5970, although when you add another GTX 480, it scales much better than four HD 5870s.
@ lower resolutions the 5970CF will obliterate 480SLI, @ 2560x 1600 or higher the 480's will be faster (also depends on game)
Bottom line, SLI scales better than CF so with this in mind it would be wiser to choose dual 480's over dual 5970's if you can take the heat and power draw. You will also save around 350.00/400.00$ if you decide to go with the 480's.