is GTX680 already exist..?
forget about rumor..
according to the GTX580 SLI requirement, it need 720 watt for the whole standart system (non oc)..
if you use water cooling, probably you overclock it.
then 850 watt is not enough to serve all of your component..
well, FPS power always increase beetwen graphics card series, but power requirement and consumption never decrease..
Go with 1kw PSU then..