I think I saw your thread on Creative Cow. I don't have a good answer to your original question, but hopefully this will help as well:
http://eriknaso.com/2015/03/21/upgrading-the-processors-on-my-early-2009-mac-pro-at-a-bargain-price/
Both Erik and Marco (previous link) are video guys using cMP. IIRC, Premiere supports both OpenCL and CUDA. Based on your other thread, I assume you are leaning towards a nVidia card. I just picked up a GTX980 for one of my cMP's and while I haven't put PPro CC through it's paces yet (I'm a novice at best), I've been quite happy with it.