If you believe this:
a pair of GTX 580 cards in SLI can draw over 600W. (That is for the cards alone.)
eVGA recommends a 600W PSU with at least 42A at +12V for a system with a single 580. I don't see an SLI recommendation, but that suggests 900W/67A for two.
(The AX850 is rated at 70A at +12V.)
The AX850 isn't on the certified list for dual 580s:
Short version: the AX850 may be adequate fior the system that you suggest, but it doesn't have a lot of margin.
Intel I7-3930k, Asus P9X79 Pro, nVidia GTX680, Asus PA246Q monitor, 4 X 4 GB g.skill DDR3-2133, Creative X-Fi Titanium, Corsair Force GT 120GB SSD, WD Caviar Black 1.5TB, Win7 X64 Pro