You definately want a board that has two 8x PCIe slots. I think most boards with multiple PCIe slots run 16x and 4x. Also, I would get one that has PCIe 3.0 slots. Here is a link to some of those.
I am about to build a similar system but can't decide if I want one GTX 570/580 or two GTX 560 ti cards. Micro-stuttering is the discouraging factor. Good luck.
why i should get an PCI-e x16 3.0 motherboad .. i mean all the graphic cards required 2.0 !! .. i don't know maybe in the future card will require 3.0 .. but not the GTX 6XX sires .. correct me if i am wrong ;(