A 750W will be fine for xfire or SLI (2 cards), so long as you do not intend to OC, if you're going to be OCing (either the CPU or the GPU), I'd go for a 850W just to make sure you always have enough overhead, that's also a bit of future proofing if you want to get a better card when the time comes to upgrade.
If you're just doing 2-way with no OC the 750W will work fine.
If you want to do 3-way 1000W would be best.
thanks! i will get the 850 because i know i will oc in the future but never will i get 3 cards =)