The ASRock N68C-S will only do one PCIe card. To be honest even though I like that board, you may want to think about a board upgrade before laying down the cash on a 6950. At the very least make sure that the board will support that card. I have read posts about it being persnickety concerning the top of the line cards.
As far as the PSU goes, go the Rosewell Capstone line is a good deal on newegg right now with 450, 550, 650, and 750 choices.