+1 I agree to this. THOUGH If you are going for SLI I would get Two 9600gt cards.
Unless you wanted to run Crysis at ultra high settings then I would recommend Two 9800GX2 cards as they are the only things that can do it... (one 9800GX2 can do it but at only at about 23FPS)
I think about 512 something watts each... just get a good PSU... they aren't that expensive... and on the bright side.. once you have one you don't need to get another
I think 850w will run 2 in SLI fine though you may want to check on that.
new idea... SLI your PSU, lol
I wouldn't recommend getting a 9800gx2... the 9600gt cards are more than enough to power the latest games and future games easily... Crysis is just so odd making others have the goal of wanting to play it on Very High for sport...