I'd personally go for the 850w, just so you have plenty of juice left in reserve, in case you wanted to add more of this n that.
but also note though, that you may decide against going for dual cards? or could you go for a 7970? they aint much more than a 7950, but are much more powerful.
Im looking for budget cards that I can fit in a spec thats around £950 and crossfire it later, when it struggles. I cant fit in a 7970 - I even tried to fit a 670. But after looking at benchmarks of the 7950 it looks like a decent card.
shame you cant fit a GTX 670 because if its two cards your after then your better off with two GTX 670s - crossfire has tons & tons of issues compaired to SLI ( to many to mention )