Pro/E is multi-threaded, but not at all times.
If I recall correctly, it multi-threads during retrieval and tessellation (for shaded images).
I suspect that for most people, that will not be very significant.
However, this does not mean that you should avoid multi-core CPU's.
They are advantageous for several reasons.
The bottom line is that you need to know which CPU's truly make a difference for Pro/E.
And the best place for that info is http://www.proesite.com.
Specifically, go to the benchmark results page here: http://www.proesite.com/cgi-bin/ocusb5.cgi?64bit=1.
As you will see, the Core i7 is best these days.
However, for the best bang-for-the-buck, I recommend a Core-2-Duo, such as the E8500.
I am currently conducting a thorough benchmark exercise to answer that same question as yours within my company.
Preliminary results show that for part regeneration, more than dual-core is a waste, and that Xeon cpu's regenerate %3-5 slower than Core i5 desktop cpu's given the same clock speed. Furthermore the Core i5 is available at higher clock speed than Xeon, for a fraction of the Xeon cost.
Furthermore, Intel's new Turbo technology appears not to help ProE... but I'm still investigating that.
Lastly, Intel's Nehalem (integrated memory controller) architecture shows significant improvement in ProE vs. all previous architectures.