GTX780 is a revelation.
Uses same power as a GTX570 and runs circles around all previous Mac GPUs. Have a look at the After Effects Benchmark thread. The 780/Titan leave all the rest for dead. I would include a 7970 for amusement but of course it can't do Ray Tracing in CS6 because it can't run CUDA.
Weeks before nMP comes out, it is already pro-obsoleted. AMD has had to drop price of the 7970 that Fire-Pro 9000 is based on below $400 because no one in their right mind would buy one with Nvidia's cards so far ahead.
This is why we bought Mac Pros with PCIE slots. Because we can make our Mac Pro current again while the suffering iMac guys get stuck with whatever shipped in their iMac. Have a read through iMac section, lots of wishing they could upgrade their GPU. Be glad you don't have your GPU locked in by Apple like those poor guys.
I am going on a trip to Bay Area but before I go I will be sending a 780 and perhaps a Titan to Barefeats. Prepare to be amazed.