To me it seems like a stupid question, of course it will be able to power your build. No offense mate.
Edit: the one 'problem' you might have is needing to daisy chain some of your fans, as the Corsair HX850 has twelve 4-pin peripheral (molex) connectors, to me it looks like you'd need 13, but daisy chaining is easy and not really a problem at all.
Well I figured a stock 570 eats about 215W, so an overvolted card would eat 250W roughly, times that by 2 = 500W, Plus a highly overclocked CPU = 200W roughly. So thats 700W
+ Mobo, RAM, HDD, Fans, Pumps.... thats surely another 150-200W which gives a total of 850-900W max load.
Its an expensive rig, so to me its not a stupid question.
That's on full load, so you'd have to be stress/torture testing on both the CPU and GPU to get anywhere near that and you'd have to be doing it 24/7 to start to have problems with something as high quality as that Corsair.
But, before I just decide to upgrade the PSU, I think I'd try it with my current PSU.
The PSU wattage doesn't matter all that much. There's a vast difference in how PSU's are rated. Some can meet their claims, some can't. This is why I believe most recommendations on PSU wattage are overestimated a touch.
The HX850 is very conservatively rated, its stronger than many 1kW units on the market, its more powerful than the Antec CP-1000 unit, so the wattage rating on the unit doesnt mean much. That Guru3d review has the power measured at the wall, and has 2 GTX 570s pulling 410W from the wall which on an 85% efficient unit is about 350W from the PSU which seems about right.
The HX 850 is a very strong unit for an 850W unit and will be plenty for that system, hardware secrets was able to pull 1000W from it with it staying above 80% efficiency and in spec before it shut down, you have plenty of power available.