I would focus on two:
Radeon HD 7950 and the Nvidia GTX 680. I've owned both for my 2009 MacPro and they worked great. There are lots of pros/cons for each, but in the end I preferred to stay in the CUDA world of Nvidia, even though the 7950 will also give you acceleration through OpenGL.
[Moved to Hardware forum.]
Looking for at least the top two video cards that are tried and tested that would work best with at least Premiere Pro and AME.
Are you adverse to using aftermarket cards that have been converted to work properly in a Mac? If you aren't, then I'd recommend talking to Dave @ MacVidCards because he can cook you up an nVidia 700-series card that will boot properly in a Mac Pro, run at PCI-E 2.0 speeds, and work wonderfully in Pr/AME.