First of all, here's my hardware setup for this machine:
MSI 890FXA GD70, 6core, 1000w OCZ
3x XFX 6870
1x MSI 6870
I finally got this running in windows 7 with 4 cards. Before you linux guys jump on me, I tried linuxcoin out, and I had 2 problems. I couldn't underclock 6870 memory, and when I went from 3 to 4 cards, linuxcoin wouldn't start Xwindows.
Windows wouldn't work for a while either. First, with 11.6 drivers, guiminer wouldn't start, Afterburner wouldn't start. I went to 11.7, which was nice, because it allowed remote booting, restarting the miners without monitors (dummy plugs have never worked for me), and the CPU bug was "fixed" - 1% CPU usage on 3 cards. But I could only start 3 at a time. GPUz wouldn't start if 3 cards were mining. 4th card also wouldn't start, no matter which 3 I started first.
I finally fixed this by uninstalling the drivers, deleting the contents of this reg key: 4D36E968-E325-11CE-BFC1-08002BE10318, and reinstalling 11.6. CPU back to 100%, back to monitor cable swapping, but it runs 4 cards.
I noticed overclocking is tricky. You have to switch between Trixx and Phoenix Rising to get the memory down, and use GPUz to check for success. Afterburner still wouldn't run. But GUIminer starts and runs now. Stable for an hour now. Power usage is 730W.
Been a long road...I hope this helps someone. Deleting that reg key was a breakthrough. Before that, going back to 11.6, win 7 wouldn't recognize all 4 cards. Driver sweeper doesn't delete that key.