I've plugged a 7970 into a seasonic 400x without anything else plugged into the psu and here are the results straight from the wall: (all settings done with afterburner - REMEMBER these numbers are only for one card not the system)

STOCK

core volt: 1.112
core clock: 925
mem volt: 1.600
mem clock: 1375
idle: 26.5w
load: 245w

MHash/s: 550

===============================================

core volts / core clock / mem volt / mem clock / load / mhash - mhash per watt

1.112 / 925 / 1.500 / 685 / 238w / 550 - 2.31
1.112 / 925 / 1.500 / 340 / 230w / 547 - 2.37
0.975 / 925 / 1.500 / 150 / 152w / 540 - 3.55
0.950 / 925 / 1.500 / 150 / 140w / 540 - 3.85
0.925 / 925 / 1.500 / 150 / 131w / 540 - 4.12
0.925 / 925 / 1.500 / 340 / 132w / 545 - 4.12
0.925 / 925 / 1.500 / 685 / 135w / 548 - 4.05
0.918 / 925 / 1.500 / 150 / 128w / 540 - 4.21 [MOST EFFICIENT]
0.918 / 925 / 1.500 / 340 / 130w / 545 - 4.19
0.900 / 925 / 1.500 / 150 / 000w / 000 [FAILED]
0.925 / 950 / 1.500 / 150 / 133w / 555 [FAILED after 30min+]
0.937 / 950 / 1.500 / 150 / 139w / 552 - 3.97
0.937 / 950 / 1.500 / 340 / 140w / 559 - 3.99
0.937 / 975 / 1.500 / 150 / 142w / 569 - 4.00
0.937 / 975 / 1.500 / 340 / 000w / 000 [FAILED]
0.943 / 975 / 1.500 / 150 / 144w / 570 - 3.95
0.943 / 975 / 1.500 / 340 / 146w / 575 - 3.93
0.943 / 1000 / 1.500 / 150 / 000w / 000 [FAILED]
0.956 / 1000 / 1.500 / 150 / 152w / 584 [FAILED after 30min+]
0.962 / 1000 / 1.500 / 150 / 146w / 584 [FAILED after 30min+]
0.975 / 1000 / 1.500 / 150 / 155w / 584 - 3.76
0.975 / 1000 / 1.500 / 340 / 158w / 590 - 3.73




I'm going to do a few more different combinations until I find one that produces the best hashes for watt. I'm gonna add more so keep watching and if you want me to try a combo let me know.