You don't have to feel too sad. even the 2009 mac pros have rather limited choices if we assume only official Apple supported graphic cards.
But I can understand how you feel the lack of ability to run better cards in Bootcamp. You are limited by the PCI Express 1.x. Most cards these days has an entry requirement of 2.0.
Apple appears to allow users to upgrade their pro desktop systems, but they want to dictate the general users what they can upgrade without cannibalizing new sales. Very in line with most companies do these days - not-built-to-last. A limited lifespan. Gone are the days where we can use an Apple Pro system for years without suffering some sort of hinderance.