I would think the 850W will be more than enough until you start LN2ing both GPUS as well

Even then it might be enough...
I am running a 750W corsair for a 920 (not DIced yet) and a beefy 4890 cold air benched

Had problems with a low Q 650W though.
I am not a fan of corsair PSUs overall... but they get the job done. My complaints are with the connectors they use (oversized IMHO broke a fan controller and thought I was going to break a video card with two different units)