A pair of GTX470's need about 28 - 30 amps. Figure another 10 amps for the CPU. That's about 480 watts. Figuring two hard drives and an optical, along with the motherboard and RAM, I estimate (high side) a total power consumption of 600 - 650 watts.
A Corsair 750TX would be enough. I'd put in an 850 watt PSU if it were my system though.