I have a system with dual 7970's and it pulls about 600+/-watts at the outlet. That includes AMD Phenom II x6 CPU, dual SSDs, and RAID1 2TB mass storage. Using GUIMiner v2012-02-19 I get about 1Gh/s combined, each individual card does about 500Mh/s. However I do not think AMD has the drivers tuned yet for these cards, nor do I think I have GUIMiner exactly optimized for this system.
I'm running dual 7970s as well (see
this post for details).
I get 625 Mh/s from each card stable at 80-85 degrees C with these cgminer options (latest AMD drivers and SDK):
cgminer.exe -O u:p -o p2pool.thedimi.net:9332 --submit-stale -g 1 -k diakgcn -v 2 -w 256 -I 7 --gpu-powertune 10 --gpu-memclock 550 --gpu-engine 1100 --auto-fan --temp-target 83
These XFX-made 7970s don't have the best airflow, in my opinion. I should have bought dual-fan ones... Then I would have been able to get 700+ Mh/s out of them!