Try this link: http://www.antec.outervision.com/PSUEngine
, Input your PC info and see how many watts it recommends. The 6850's are not listed yet as the are so new, I used X-Fire 5850's which draw more power just to be safe.
I used high end desktop, your CPU, RAM, 1 DVD rw Drive, HDD, soundcard, 4 USB's, 4 Colc Cathodes, 4 120mm fans, 20% cap aging. That is with no OC keep in mind as I dont know the numbers to enter, so you will be higher than the reading below.
And got 596w so you should be OK with the Corsair 650w, they are usally underated from the amount of power they really can provide. Now if the 750w isnt much more go with that just for extra headroom.