Amazon currently has an excellent deal on a Cooler Master 1000 watt gold-certified fully modular power supply. It’s $125 after a $30 mail-in rebate, which makes it about $75 cheaper than its normal price (new units on eBay typically sell for nearly $200). This PSU is reportedly a re-badged Seasonic and should have no problem providing efficient power to a rig with four undervolted 7950 GPUs (or three full-voltage cards).
Update: Looks like the deal has ended. The rebate is still available, but Amazon has raised the price back to its normal level. Hopefully some of you were able to snag one!
Update: The deal is still valid, although it looks like it’s been pretty popular, with shipping estimates in the 1-2 month range.