Yes I'm sure. This is common knowledge that the 1st gen mac pros only have 32 bit EFI, so you cannot put 64 bit windows on it natively. No need for sources, practically everyone that has a 1st gen and tried to upgrade their video cards discovered this unfortunate fact. It caused quite an uproar in 2008 when they released the 8800GT and said that 1st gen users couldn't use it due to the EFI problem.
As far as the bottle neck, I'm not entirely sure if it's the processor, the slow ram, or the architecture, but if you head over to barefeats.com you can see that any video card above the radeon 4870 performs the same on a 1st gen mac pro, thus something else is slowing things down somewhere because the newer cards don't provide any performance increase.