I had this problem where i could not get windows 7 to show 3 of the 4 5850s in device manager. i checked the bios, reseated the cards, tried one card in each slot which worked, but i could not get all 4 to show in device manager. I previously was running this box with one 5850 and it ran fine with cgminer so i knew the config was right (SDK drivers...) This is really weird because windows upon boot checks for new hardware and i even scanned for new devices which doesn't really do much after boot.
It ended up being an ATI thing. i re installed the 11.11 driver package and all 4 showed up in device manager. So it anyone is struggling with cards not showing in device manager run the ATI driver package even if this does not make sense to everything we know about windows hardware.
910 watt PCPower and cooling
4x 5850s OEM (bensoutlet)
120 5400 hd
couple of 120mm fans