*GTX 770 SLI Question*

From a quick Google it looks like each card can take up to 230w without OC so that's 460w, then if you OC you should allow an extra 20% bringing you up to around 550w. That's leaves 200w for the rest of the system, which should be ok with a good quality PSU but with such a small price difference between 750 & 850 why get a PSU that's at times might have to work very hard, the more stressed a PSU is the more likely it is to fail or at least not last as long.