Idle: ~130W (0 MHash/s)
2 x 925 MHz: ~670W to ~680W (1104 MHash/s) - Power use increases as temps increase
2 x 1125 MHz: ~750W to ~790W (1345 MHash/s) - Power use increases as temps increase
Idle (with OC): ~130W (0 MHash/s)
I'm guessing that a slightly higher rated power supply with better efficiency might be called for on this one.
You could probably shave another 40-60W per card by undervolting. I got 50W off of mine @ 1125MHz which lowered the temps enough (8-9ºC IIRC) to afford running the fans at slower speeds, and others on the forums have been able to undervolt even more than that.