I wonder if unRAID will support a hackintosh if you were to use Apple hardware? The MSP I used to work for sells hosted VM's/VDI and we were going to sell hosted MAC VM's because VMWare support virtualised MAC's ONLY if it's hosted on Apple hardware. VMWare will allow you to run a dedicated physical GPU for a VM, though I've never tried a consumer GPU on a VDI, I mostly only get to work with Teradici PCoIP GPU's.
On a side node, there is a company called Sonnet who sell a 4U rackmount kit that can hold two of the "trashcan" mac pro's.
