around 580 - 1x Lancelot FPGA + 1x 6850 running at the moment. Got some issues with my 2x 6870 Cards sitting in my rig right now, cant get them to do more than 25-30MH/s .. 
The problem is your drivers.
Easy fix; uninstall the entire amd suite through the catalyst control panel uninstall in ad/remove or whatever.
Restart the machine and reinstall.
I have to do this every once in a while, whilst tuning a rig.