I recently moved up to an Athlon64 X2 4200+* from a P4 3Ghz Prescott** and I do see slightly improved rendering times in Poser6.  Granted some of the improvement could be from an overall improved system.  Although I had hoped for more, the real bonus is that I can do other things while rendering which I could never do with my single core P4. I can easily run photoshop and other software at the same time (during rendering), with no slow down. 
While the rendering times aren't much shorter, the fact that I can do multiple tasks at once increases my productivity and effectively cuts the overall time to do work. 

So, in a nutshell, yes it was worth the upgrade.

*New System:
Athlon64 X2 4200+, 2GB PC3200, GeForce 7900GT

**Old System:
Pentium4 3.0E (Prescott), 2GB PC3200, GeForce 5900XT