1- What is ur budget for the mobo? some mobos can run Crossfire at x16 but are really expensive (like the Crosshair III or Gigabyte 790FXTA-UD5)
2- Here are two very solid mobos for ur requirements:
a- ASUS M4A79XTD EVO
b- Gigabyte MA790GPT
c- The Maziar
option looks good too, but I can't find if in Crossfire run at x16, x8 or x4.