all on the same rig
PhenomII @ 1GHz, PSU is the same SF 80+ gold as used before

5970, linux amd64, cat 11.12, sdk2.5
idle @ 0.95V/157/1000, 182W
idle @ 0.95V/157/300, 152W
idle @ 0.95V/157/150, 149W
cgminer @ 1.05V/820/300, 377W, 761Mh/s
cgminer @ 1.05V/600/150, 308W, 542Mh/s
cgminer @ 1.00V/600/150, 291W, 542Mh/s
cgminer @ 0.95V/600/150, 273W, 542Mh/s

7970, linux amd64, cat 11.12, sdk2.6
idle @ 0.85V/300/150, 115W
idle @ 1.175V/300/150, 132W
idle @ 1.175V/1150/1070, 168W
phoenix @ 1.175V/1150/1070, 363W, 676Mh/s
phoenix @ 1.175V/1150/1375, 370W, 676Mh/s
phoenix @ 0.85V/300/200, 154W, 178Mh/s

7970, windows 7, cat 11.12 7970 edition, sdk 2.6
screen off for a bit,102W
idle @ 0.85V/300/150, 114W
idle @ 1.175V/925/150, 142W
idle @ 1.0V/925/150, 128W
phoenix @ 1.0V/925/150, 235W, 535Mh/s
phoenix @ 0.95V/925/150, 222W, 535Mh/s
phoenix @ 0.92V/925/150, 217W, 535Mh/s
phoenix @ 1.175V/1150/150, 328W, 659Mh/s
phoenix @ 1.175V/1150/300, 339W, 666Mh/s
phoenix @ 1.175V/1150/1070, 368W, 674Mh/s

2MB PCI S3 VGA card, linux
idle 101W

so let's run some numbers, using 0.95V as undervolt for 7970...
whole system:
max OC @ stock V: 5970 2.02Mh/J, 7970 2.01
undervolted: 5970 1.99Mh/J, 7970 2.41

assuming system is 100W (well, more or less...)
max OC @ stock V: 5970 2.75Mh/J, 7970 2.89
undervolted: 5970 3.13Mh/J, 7970 4.39

bullshit calc (aka simple load - idle)
max OC @ stock V: 5970 3.38Mh/J, 7970 3.07
undervolted: 5970 4.37Mh/J, 7970 4.95