750w should be sufficient, even with overclocking if you get a good PSU. There's stuff like this:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139021
Goes over slightly by $9 (well, under by $5 with rebates). At max it can pull 760W on the 12V line and around 900W total (1,100 from the board!).
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371024
This is a fair bit over but if you can get a rebate it goes under. I'm not sure who the original manufacturer is though... I think Delta? On the 12V rails you get a max of 696W and has a lower efficiency so uh... corsair?