P55 if u don't need all that multi-processing. Otherwise X58. 2 PCIEX16 stolts 2.0 X8 will not hinder crossfire performance more then a few % even with a beasts like 5870's. But look for a P55 mobo that actually can deliver X8 on it's 2nd PCIEX16 slot. For GigaByte those boards are labeled UD4 and higher (UD5, UD6, ...)