The cost of four GTX TITANS will be less than two TITAN Z, which is another reason to consider the TITAN Black SLI over a TITAN Z SLI.  In addition, it would seem that the TITAN Z would work when there is limited room for GPUs and SLI is not possible.  Two Hydro cooper TITAN Z cards will run $7,776 (from Amazon) which 4 TITAN Black cards will run $4,400.  Even with the water loop and blocks, it will be considerably less than the two TITAN Z cards.  Seems to be a bit of overkill even in a folding rig.

Counteragent
Hi guys,
 
So my question is if you did purchase 2 x GTX Titan Z's and run them on a SLI m/b, would all GPU's run at x16? since you're only using 2 x PCIE sockets?

In SLI, the GPU's are rendering individually and are passing data between them via the SLI bridge, so functionally as only one GPU per card is actually rendering at a time, PCIe bandwidth shouldn't be affected. In other words, for SLI, you wouldn't be cutting your PCIe bandwidth per GPU in half. On the other hand when you're not using SLI, applications that can take advantage of the computational power of the GPUs in parallel, then PCIe bandwidth might be an issue. If you're running on a PCIe 3.0 board, that would still leave you with the equivalent of x16 2.0 on all four GPUs. Whether or not that would affect end performance would depend on if the application(s) are able to saturate the PCIe bus at whatever speed you're running at.