Despite being a dual-GPU card, the HD 5970 actually consumes less energy than the GTX 480 "Fermi" card.
What PSU would you recommend to fully power Two of them in CrossfireX as well as the rest of the system and then some?
According to AMD you would be good with a 850W or better PSU: "850 Watt with two 75W 6-pin and two 150W 8-pin connectors for ATI CrossFireX™ technology in dual mode".