Guru3D was able to get their test system to pull 719W from the wall outlet with two GTX 580s in this system (source
). So if I assume 85% efficiency during this power draw, then that would mean that the system pulled about 611W from the power supply.
So if I add 150W for the CPU and other goodies, then that means we could be looking at about 760W of power being pulled from the power supply under the highest loads when both the CPU and the GTX 580s are under full load at the same time.
If I assume about 80% efficiency, then that would mean the system pulled about 575W from the power supply which would be about 725W after adding the 150W.
On the other hand, if I assume 90% efficiency, then that would mean that the system pulled about 647W from the PSU which would be about 797W after adding that 150W.
So a quality
750W power supply is the minimum, but I would go with a quality 850W power supply just in case you get the urge to overclock.