Especially if overclocking I would have to agree with the above that go with the 850W.
At least your machine will run closer to the optimal range ( 50-75% ) when fully loaded.
I have the HX850 in two machines, and it is great. So if you decide to go with the 750W the HX750 will be really nice.
But if you go with 850W, I can warmly recommend the HX850.
High 80+ silver efficiency, semi modular design, 7 year warranty excellent test results.