Under Linux, I have 6 Nvidia cards working.  Not sure what is up with the Proliant G7 series.  I have tried a DL360, DL380 and the DL580 and none of these will boot with more than 4 GPUs installed.  Windows will lock up at loading screen.

Slap a USB stick with HiveOS on it and 6 get recognized easily.

I have a ML350P G8 that has taken 6 GPUs in windows easily.  

Edit* Make that 9 1060s in one 580 in HiveOS.