This guy got set up for brute force hashing with 4 hd 5970's. He has them all in 1-U rack but not mounted on mobo. http://blog.zorinaq.com/?e=42
The 5970's are separated off to one side using PCI-e 1x extenders that are modified to re-route power so that none of GPU power does go through mobo (instead of about 30-40%).http://blog.zorinaq.com/?e=44
Saves money running more units off 1 mobo ...
Also mentions that the HD 6990 maybe coming out with about 28% more hash power than 5970, anybody confirm this?