Why do people respond to questions like this one (and this happens all the time, btw) with "I'd go for a 1000W unit!" as if the OP is asking what PSU wattage he should buy here ... like the guy doesn't already know that a 1KW unit would be a bit more optimal for his setup
The OP has stated that he already has an 850W unit, and doesn't want to buy a new one
if the one he has will suffice.
Now, to answer the question ... yes that PSU will totally work for your proposed setup, esp. if you're not going to OV/OC the hell out of the cards, and then trying to run like Super Pi and Furmark at the same time.
Once installed, what I'd do is run some games, and do the following:
1) Buy a kill-a-watt and monitor you wattage usage, and remember to multiply that number by .85 to figure the actual DC draw. If .85 * (wattage on the kill-a-watt) is > 850W, I would become concerned, and
2) Keep an eye on the droop on your 12V+ rail using your favorite monitoring program ... if you're dropping down below about 11.5V at load I would become concerned.
Worst case scenario you won't be able to OV/OC the gpu's as far as you otherwise might. If I were you I would NOT drop your CPU OC to compensate though. You're much better off compromising some GPU clocks if necessary. You def. want as much CPU power as possible when running SLI 580's, IMHO.
I would certainly TRY w/your current unit. Worst case, your PC will simply shut off when you put it under a heavy load ... then you'll know you either need a bigger PSU, or that you need to find a way to lower wattage consumption. Also I wouldn't 'create' an artificially high load by running CPU and GPU torture tests at the same time ... this is a totally artificial consumption profile you'll never see in regular usage.
I ran GTX480's on my current rig for awhile, with small OC's (like 800/2000) and no OV, and it ran totally fine. I think you'll find that corsair 850W can handle what you're talking about Edited by brettjv - 10/8/11 at 1:30pm