CUDA programming on Mac Pro or alternative with ATI 5770?

Get a second card - GT 120, or just program in CPU mode.

 

Correct, you will need an nVIDIA card to use the CUDA programming model!

I'm considering a Mac Pro to program in OpenCL (less mature than CUDA at the moment, but maturing quickly and will likely overtake it in the long run).

OpenCL can be run on nVIDIA, ATI, AMD, Intel and other hardware. It may not be the best choice now, but be aware of it for the future.

 

There's not much evidence of OpenCL stopping falling behind CUDA right now, let alone catching up or overtaking. Look at all the tools available for CUDA and the weak state of OpenCL numeric libraries.

For the OP I would suggest seeing if you can find someone with a GTX285 Mac edition to sell. You might also try using a 120 to help a PC card boot with the netkas injector. If you toss the Ati card you could in theory use a 120 to boot and do CUDA off a PC 580/480, or perhaps a PC 285 if you do not want to fiddle with the power supply too much.