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/7sqrzKeep in mind that the draw from the wall will be about 20% or more greater than the DC load on the PSU. That 750W number is what it is capable of delivering in DC. At 750W at the wall, the PSU may only be delivering around 600W to the hardware (more if the efficiency is better).
Around 675, iirc their peak efficiency is above 90%. So ~610 watt at the meter is what I'd consider safe.