I have 2 6870s, 1 5850, 1 7950, 1 5670 (slot used to be occupied by a 5970 until it died), 1 BFL FPGA Single, 1 MMQ, and 4 X6500s  on 1 rig running BFGMiner without issues. All of my GPU cards were made by Sapphire.

Win7 x64
13.1 & 2.8 (1084.4)
Athlon x4 620 on a Gigabyte 890GPA-UD3H
Using IGP as primary display
Corsair AX1200 PSU