Got nothing really to add, since it's been covered admirably. I can say that a Xeon-based system is a solid choice to go with. The ECC RAM ensures fewer memory errors, the CPUs, as mentioned, are made for greater parallel operations, the whole thing is built for heavy loads and longevity. You can always get an external GPU to plug in and do the 'oomph for GPU-based render engines (I'd flirted with that idea for my laptop, until I found one with a GeForce GTX built into it), or if you have the slots for it, I bet you can still buy and install a decent GPU/vidcard internally later on.

BTW - did you get a built-in RAID controller onto the motherboard?