Apple is taking ages to update the macpro.. even so to the point that there's a ton of speculation that the line is going to be discontinued.
For me, I love my MP. It's the best computer I've ever owned but that being said, I'm on the brink of going back to PC due to the lack of updates from apple and their tendency to turn their backs on pro users as of late.
It's true that the video card is key in 3D graphics and depending on the sophitication of your 3D scene processors are key also when it comes to rendering.
What app do you plan on using for 3D? Keep in mind that there's a bit more 3D options in a PC over a mac. Maya is still a bit unstable from my expierence and 3DS is not really an option unless you use bootcamp with windows and that can be a headache if you need to switch between the two. (a virtual machine is not really an option since it will steal resources that you need while working in 3D).
Plus another benifit of a PC is that you can buy a totally badass system for half the cost. You could probably buy a pretty effective 3D workdstaction and have a little money left over a macbook air or pro. This is what I would recommend.
You and I probably agree that OSX is a much cleaner OS over that fugly windows interface but the reality is that the 3D community favors the PC market over Macs.