The OctaneRender Team pointed me a posting on a Two EVGA GTX 680 (3,072 Cuda Cores) + 450 watt PSU rendering system in a MacPro:

http://forums.macrumors.com/showthread.php?t=1520765

Exactly what I've been looking for. (I think?)

Thanks Chris
Looks like there is a power problem with two 680's. So, how about a lower end NVIDIA card to drive the 27" LED display and a single 680? The 680 would be used for CUDA processing but connected to no display?

I received a response from EVGA that seems at odds with info in Otoy’s FAQs:

EVGA:
Is the mother board on that Mac SLI compatible? For 2 cards in SLI the second card only helps in processing and does not use any of the VRAM so 2 2GB cards will only use 2GB. I would recommend just using 1 680 in ...

Reading This:
Octane Render completely relies on the GPU for rendering performance and scales extremely well. If your motherboard can accept more than one video card, adding additional video cards will greatly improve Octane's rendering speed because Octane's performance scales perfectly with the ...

About to purchase Octane for Rhino. Running both through Bootcamp.
I currently do not have a nVidia/CUDA card and looking at the 680 for Mac. . .

A couple of nuub questions:

If a single card is not enough power I should be able to just add another 680 for Mac right?
Would I then need additional ...