SLI with a 580 will be BEAST. that's what I'm gonna do when the time comes and I can afford another 580. I can imagine it running BF3 max settings at a decent frame rate (even though specs for max setting haven't come out yet.) the BF3 trailers were recorded with a single 580 and they're damn sexy..
also make sure you have good enough cooling etc in the case for two 580s!
but what if their other parts require less power? Also the PCspecialist site said his setup with SLI would need 888W, and if it were me I'd trust the site that will be making it for me.
hm.. well i may get the 850w then :S but i'm not sure! if i don't get as much money as expected i may have to get the gtx 570 :/ but would definitely sli that in the future :L
well just because im running the HX1000 doesnt mean it snecessary, I dont honestly know how much power it needs tbh. Id go for what PCS tell you to be safe, I think you would be fine with the corsair 850, but I dont really know enough about power consumptions tbh.