I have heard the Corsair power supplies are very good, efficient and stable.
But, personally i would go with a PC Power & Cooling Silencer 750 Quad (Crossfire Edition)http://www.newegg.com/Product/Product.aspx?Item=N82E16817341011
I bought one for a friends build, and I love it. We are running a Q6600 @ 3.6Ghz (50% OC) and a 2900XT (which is a juice hog), and it is super stable!
Maximum PC even used it in there 2007 "Dream Machine." And they have no budget for the "Dream Machine," so they could have used any powersupply they wanted!http://www.maximumpc.com/article/dream_machine_2007
PS. It also will match the 3870 X2's, and costs about the same as the Corsair you mentioned (without the mail-in-rebate hassle).