Not powerful enough. I have 2X 7970 and my total system power at stock levels while running benchmarks exceeds 600watts. Currently using an old Pc power and Cooling 750Wat Silencer PSU with a single rail of 60Amps
You may be able to get away using a single 7970 but I wouldn't think of OCing
i think 750 would even be overkill 0_0