Well, the thing about the Mac Pro is that it's the only Mac that uses Nvidia cards. That's somewhat smart of Apple, because it gives added appeal to the Mac Pro. However, that comes with a catch in my opinion. It reduces compatibility with new cards, because Apple needs to put them in themselves unless you're the master of Mac. If this is confusing to you, it's somewhat confusing to me as well. I'm just stating what I know.
Apple has to put them in? I would think the Mac Pro is as close to a PC tower a mac will get....don't you just slot them in somewhere