[bobkn]

If you believe this:

http://www.geeks3d.com/forums/index....ic,1946.0.html

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:

http://www.slizone.com/object/slizone_build_psu.html

Short version: the AX850 may be adequate fior the system that you suggest, but it doesn't have a lot of margin.