If it's a dedicated mining rig try LinuxCoin (
http://forum.bitcoin.org/?topic=7374.0). I've been using it for a while and haven't had any problems.
You should be able to use 4 GPUs in windows. You can use 8 in linux.
I'm running 3 cards without crossfire in a windows 7 x64 rig (2x5850+1x5830).
Have you tried running without crossfire?