joytech's got it right. 3 way GTX580's will pull about 60 amps - 720 watts, peaking at 70 amps. Figure 120 watts for the CPU, 100 watts max for motherboard and memory, and say 50 watts for 2 hard drives and a non-BlueRay optical.
Those are worst case, peak estimates so a good 1200 watt PSU would be enough.
I prefer to operate a PSU at around 50 - 70% of capacity, so I'd use a 1500 watt PSU.