Your PSU is an 80Plus Gold, so that's nice. However, it's not a single rail PSU. It's got 6 rails, which is bad for mining. I'd suggest THIS one. Each rig will pull ~ 1100W, and that PSU can handle that no problem.
Totaly, you're looking at ~3.3kW.
250 watts each? Sure, it's possible to do that. If you're pulling 250 watts per 7970 though, you're doing it wrong. My FIVE card rigs don't even pull 1100W. 996 watts (at the wall) for 3050 Mhash/sec (with the same Seasonic power supply). I'm confident that I could hit 3150 Mhash/sec and still keep my 5x7970 rigs at 1100W or less.
Undervolt. Drop the memory clock. Keep the temps down below 72c (huge leakage on these chips with higher temps).