Thank you for confirming that running 8x HD79XX gpus under windows is possible.
As far as I see, most people are limited by 6 or 7 gpus. Is there any tricks to do that?
There is no trick I think. Or W8 did the trick. I have not tried 4X7990 on W7, so I cannot say W7 does not support 8 GPUs.
I have 5 different types of MB, all can run 4X7990. One of them is Asus P55 chipset, no problem running 4X7990.
If you cannot see all 8 GPUs, just reinstall Catalyst. In my PCs, catalyst earlier than 13.4 does recognise 7990.