And ... contrary to what I said before, it should be just fine when u add the 2nd gpu. Just not on the "hoogy" side ... and you do not want that.
I am going to do a similar build, but w socket 1366/920 ... I'm gonna have a single 16x slot, dual GPU card and 2 (or 3) SSds and 2 (or 3) 3.5" 2TB drives so, I might go with 750W but I could prolly get away with 600W~650W, without ever breakin' a sweat.
... If you do any "audio", you might have learned that most electrical components operate "best" (in several ways) when they are operated between 50% and 70% of rated max power.
PSUs are rated for PEAK and CONTINUOUS power ... Ususally seeing PEAK sometime during the boot sequence, as the "empty" capacitance of the whole system sucks up power and as any mechanical drives are spinning up. This is a really big deal (moreso, in the past), when you fire-up a huge disk-farm type storage server OR a monster 4x SLI rig which is "loaded up" with bunches of fans, which also suck 2x more power when spinning up. If you scan Wiki, on both Inductance and Capacitence, you will see the DC power curves, showing current, in the milliseconds after power is first applied. Even the coils in the PSU have a large inductive capacity (big magnetic field that must be built up from zero).
Once the mem checks are complete and all the vearious fans, coils, and drives are up to speed, well, then things smoothe out and you are at cruise level ('cept for turbo boost or O.C.)
So ... whether you are talkin' 'bout the huge sucking power drain, at power-up, or the sustained max current, during O.C.d SLI video codec transcoding renders (which can take hours or days), you would like that extra "OVERHEAD" so the PSU will be comfortable, under heavy load, and not get really hot (which degrades efficiency and "quality" of output.
Make sense? I don't turn my stereo up to "eleven" for very long ... and every stage of my audio, from the first preamps to the vol on my powered studio monitors is tweaked to about 60%~70% ... where my signal remains "cleanest" (most pure).