Weird cuz that's exactly the layout of this thing.
8 slots, 4 usable with proper GPUs. Not good.
(proper GPUs extend to two slots)
Unless, of course, you limit your choice to single-slot cards. Something like this:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814131445Then you can get 8 of those, the mobo you said, a 1200W power supply* and your 1.6 Ghash rig is reality.
* 1200W is enough. I do 1.4Ghash with 750W on less efficient hardware.