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.
Just curious, you're driving 3 7970 with a 850W PSU???
Considering that each GPU is taking 250-300W, this is pushing the limit a little bit, isn't it?
Are you tweaking the GPUs voltage to make this setup be stable?