FCP.Co has an article on installing a PC GTX570 in a MacPro to get better OpenCL performance in FCP X, but it will also increase your CUDA power for Premiere Pro and After Effects CS6.
Still you can score over 1000 on Luxmark if you go for a GTX670 instead (which has a huge amount more of CUDA cores, and can still draw it’s power from the internal power supply instead of say a 680, which would require an external power source to run), but they are not as well supported on the Mac. Either way you will need to go to Netkas to get some instruction and some help (though some of the help can be rather surly, but it is worth it if can really beef up using your MacPro). Someone at Netkas has also figure out how to Firmware update recent MacPro’s to the latest firmware for the newest itineration, allowing beefier XEON’s into your old MacPro.
The instructions are pretty easy, but I have found that they don’t always work, so think about re-installing your NVIDIA drivers once you are done, as that did it for me.
You will also need to do some hacking to Premiere Pro and After Effects to get them working with the CUDA cores on a new video card.