If you are going to use them for gaming, having multiple GPU and just 1GB memory makes no sense.

stock: 225W per card + 95W for cpu = 770W, now add HDD+fans+mobo+overclocks+usb+misc, it gets pretty close if even higher than 850W. Even if it's left below 850W, the PSU will degrade over time and you might see results of it in 1-2years.

I would go for single 7970(their price should drop as 680 is ~equal/better with ~same/lower price), which is about same gaming performance than 2x 6950. Also it gets rid of nasty crossfire/sli problems. Or wait for 680 gtx optimized bitcoin mining results : )

As the mobo only has pci-e x1 and not x4, for my knowledge it will require some extra modding to give the card enough power. Or just a proper x1->4x/x8/16 adapter.