I have 6 cards per rig, 3 of them on powered risers... two rigs... 3 Seasonic 1250 Watt gold power supplies.  Each power supply powers 4 cards.

I have the Gigabyte 990FXA-UD5 and the MSI 990FXA-GD80 and they both can handle 6 GPU's I believe (still waiting on 1x risers to arrive).  It looked like they had been out for awhile so I didnt know if there was anything better on the market that most people were using or not.