Mac pro vs. windows maschine

i need to get a new machine, but have difficulties to leave osx entirely.
is it possible to use “windows” graphic cards on a mac pro booting in win7 ?
why nvidia over amd (eyefinity) ?

some shaders don’t work on ati cards, or at least need to be adjusted codewise.

can you stick just another graphicscard into a mac is something for a mac forum…doesnt it ?

and why buy an overpriced “pc” if you can built your own mac-pc bastard ? ;)