Tyan and Supermicro come to mind when I think of motherboards with heavy expansion. The only one I've used recently was based on this Tyan:

http://www.acmemicro.com/Product/13023/Tyan-B7059F77AV6R-2T-4U-Server-DP-Xeon-E5-2600-LGA2011-Register-DDR3-6x2-5-4x3-5-SATA-Hot-Swap-10GbE-PCIe-R2400W-80PLUS ...

OP, any time you're looking to cram that much GPU power in a single enclosure, there is tons of tweaking (power & cooling) to be done before you have a 24/7 100% load stable system.

The rig you linked is cool as hell, but for getting real work done I'd look to one of these suppliers:

http://www ...