When I said PC cards, I had two ideas: run Windows 7 64-bit, maybe you play games or do work in Windows where cards do better.
Or flash, but I didn't and there is as you say, no information yet on what to expect.
Just my say, I don't know if you meant to, but you did put words in my mouth that I didn't.
I was surprised by the problems and by the lack, of using flashed ATI's.
And I still dream of a system where you can buy a GTX 460 and use it (there is also a new 2GB model out).
Normally, you will see me point to and link to more information to pursue.
http://www.apple.com/macpro/features/graphics.html
http://techreport.com/articles.x/19342