Depending on price and availability, maybe you should take interest in the new 5000 series of video card. A single 5770 will perform close to a set of 4770 cards in XFire, and the 5850 will perform better than the 4890, but for more money. With the 5000s, you also get DX11 (but no games uses it right now).
As for the MB, I would definitely go with the GA-EX58-UD4P. Gigabyte is a good company and the board supports both XFire and SLI where the EVGA's support only SLI.