I haven't tuned the software on these rigs for minimum overhead and I haven't overclocked the video cards yet either. Currently the farm is running at ~6GH/s or ~500MH/s for each 7970. I built it as fast as I could to get the mining started so they are currently running Ubuntu desktop. Ultimately I plan to create a Gentoo USB install that automatically tunes and mines each rig.
Wow, good job! May I ask you how much dollars you invested in this?
Btw, with a 7970 you should be able to get at least the 600Mhz (Gygabyte goes to 720).