We have the same mobo, and I manage to get 4.4ghz OC. You should be fine with that mobo, until you decide to crossfire and OC by bigger margins, to which I would get a better mobo for.
Nice build overall, but you should really get a better power supply, like 750 - 800w, 600w should in theory be enough for the 280x windforce, but i have that exact card and i had to replace my cx600m with a 750watter before the windforce would run properly, (ie without BSOD, cutting out, crashing etc.)
Sorry to put a dampener on things, but since i had a near identical build i felt obliged to tell you what i had to do.