Yeah, you'll need some of those PCIe extension cables to fit them all, and the proper PSU, but yeah it will work. In the past, you needed to use Linux because Windows only supported up to 4 cards (Linux = 8 ). CGminer/AMD software might have changed this though. (Just like how we no longer need dummy plugs)
Windoze actually supports 8 GPU's, 8 GPU cores that is though, so with dual GPU cards such as 7990s you can only run 4 cards.