For max efficiency and longevity (i.e., less constant load on the PSU), I'd recommend at least an 850W, but a 1000w would be better (roughly double the wattage that your setup would pull at load, and PSUs operate most efficiently at 50% load).
Obviously, wattage only tells half the story. The +12v rail amperage is important, too, since that's what will be providing most of the power.
I'd recommend this XFX 1050w.
http://pcpartpicker.com/part/xfx-power-supply-p11050befx
$139 after rebate, made by one of the top manufacturers of PSUs, fully modular, 80-Plus Gold. Excellent unit.