Tbh, any motherboard with multiple PCIe slots will be good.  

The speed doesn't matter, you can use x16, x8, x4 and even x1 slots.  Don't believe me?  Here's my proof of concept.

https://twitter.com/relay_1/status/290159160987897858/photo/1

The motherboard is a http://www.newegg.com/Product/Product.aspx?Item=N82E16813128514
Which has 4 PCIe x4 - x16 slots and 2 PCIe x1 slots.

I'm using these PCIe risers to connect them.
http://www.amazon.com/gp/product/B005TEJ3MI/ref=oh_details_o05_s00_i00?ie=UTF8&psc=1
http://www.amazon.com/gp/product/B00646VJDG/ref=oh_details_o05_s00_i01?ie=UTF8&psc=1

Or you can slap 4 cards right on the board.

Dummy plugs aren't needed unless you're using Windows I believe.  I use Ubuntu and it's not required.  I had them on in that photo, but took them off shortly after and it still works perfectly.

Good luck.