Ok.... let's see;
i5 2500k = 95W TDP
ATI 6970 = 250W Max Power Draw Each. Since you crossfire, that's 250W x 2 = 500W Max
Mobo, Ram, HDD, Optical Drive + misc = another 150W [Somewhat of an over-estimation - just to be safe]
That's about 750W - Which is why I would recommend a Corsair 850W Enthusiast [Bare minimum]. Although a 950W would be the best choice imo.
Note: It would be quite difficult to get both cards to run at 100% but it never hurts to be safe and to have some headroom for more RAM, blue-ray drive, hard-drive(s) and SSD(s).
Corsair 850W Enthusiast ==> http://www.newegg.com/Product/Product.aspx?Item=N82E16817139009
Corsair 950W Enthusiast ==> http://www.newegg.com/Product/Product.aspx?Item=N82E16817139013
Edit: Need more coffee