You'll be fine at 750W. A little maths might clear things up. So, lets say two GTX 580's @ 500W + CPU @ 130W (Exaggeration unless you have an 8000 series FX, Socket 1366 i7, or SBE) + All other components @ <120W = less than 750.
And this is an exaggeration. Lets be honest, when is a machine like that going to be under full load? VSync will rein the 580's in in all but the most brutal scenarios. You're not likely to be under full CPU load whilst gaming either.