Asus X99-E WS is a good starting point. You can put four double wide GPUs on that.

Another really nice one is this one:

https://www.supermicro.nl/Aplus/system/Tower/4021/AS-4021GA-... (AMD based)

https://www.supermicro.nl/products/system/4U/7047/SYS-7047GR... (Intel based)

http://www.dell.com/us/business/p/poweredge-t630/pd

(That last one should be able to hold 4 GPUs but I'm not quite sure about whether or not it will be able to power all of them, Dell isn't helping with their documentation either.)

Edit: just found this:

http://www.supermicro.nl/products/system/4U/4027/SYS-4027GR-...

About $4K, + 8 GPUs that's a pretty penny. Drool.