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.
Just my 2oz of copper.
Thanks,
The 1911Moocher