No way they both need 500W. 850W should be plenty.
I would recommend against this build though. crossfired on that mobo is x16/x4 so the speeds are severely reduced.
i would go with an i5 -2500k, asrock extrem4, and a corsair 850w v2. then xfire those 6850s.
The 16gb of RAM is overkill. a 2 x 4gb 1600 mhz config is plenty for now and leaves slots open for future expansion.