dont know if that would just be unnecessary overkill. that being said the 850 is very well reviewed, and its modular. with the ram combo its actually cheaper than the xfx 750, but still like 30 more than the corsair 750 and ram combo
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.