Just a little personal workstation experience. I run a Dell 530 Low End workstation for PRO/E with the following:
Dual 1.5 GHz Intel Xeon Processors (Not necessary PRO/E is designed for single processor machines)
1.5 GB RAM (Worth it, see below)
NVidea Quadro2 Pro Video card (3rd card is the charm, I already burned up 2)
This is a three year old machine on which I manipulate 3-d models of power plants with few issues. When I got it I had 756 MB of RAM and had trouble. Since I got the upgrade I've had no trouble. The additional processor speed on newer machines doesn't make a big difference in their performance like the RAM. The NVidea is impressive but when I fail that about once a year it makes me leary of them. As it is the IT department always wonders what I will call them with next
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As it is I can run the student version of the software at home on my PC (1.8 GHz P4, 500 MB RAM) and the limiting factor for model size is RAM.
BTW I also do the occasional photorealistic render from CAD on my workstation as well.