2 x Sapphire Radeon 7970
Corsair 750W PSU
I'm guessing that a slightly higher rated power supply with better efficiency might be called for on this one.
No shit. Corsairs are rated at continuous wattage, and 7970s use 250w each at stock, and you shouldn't exceed 90% of that 750, and you're doing 750-790 at the wall? Holy crap dude.
Go buy one of these:
http://ur1.ca/7sqrz