^+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.