I wouldn't suggest using that PSU to power dual 280x's. Since you need a new one get 850w preferable from Seasonic, XFX, Antec, or Corsair(AX, HX, TX series).
Well the cards should run in crossfire you just need to make sure they will fit in the case, the PSU is of sufficient size and quality, and the motherboard supports crossfire.
What model of Cooler Master? Cooler Master has a couple good ones and they have a few bad ones. Also what are the rest of the system specs? Even a good 750w PSU might be pushing it.
I wouldn't suggest using that PSU to power dual 280x's. Since you need a new one get 850w preferable from Seasonic, XFX, Antec, or Corsair(AX, HX, TX series).