M4A79T isnt as good for crossfire.
My recomendation is the M4A89GTD Pro/USB3 if your on a budget. Great reviews, great board with great features. Its nothing too high end. Its not the 890FX chipset but it wont bottle neck the crossfire for the 6850's.
Ive been looking a lot into ASUS boards because Im building a computer and I have been looking at crossfire and a medium-high end PC. The way it came down is this:
M4A89GTD for reasonable price, good all around support for crossfire,USB3, Sata3, general use and little overclock here and there. Its good to resell, also, because it has the option for onboard video incase you want to upgrade later and just make a HTPC or whatever else you want to do.
M4A89TD for high end crossfire and better overclocks and such. Just a more high-end and gaming oriented board.
Crosshair Formula is the good stuff but for a good price. Shouldbe self explanitary. I dont think you are going for this because your build, so far, consists of decent parts and this mobo would be overkill I think. Its a great board, but more for watercooling and the higher end cards crossfire and such.