Any Poser version will run on a dual or quad core CPU, though only Poser 7 and higher will make use of the extra cores.
Poser 7 can utilize all 4 cores when rendering, speeding up the render quite a bit.
Only certain stages of the render benefit from having multiple cores though - loading textures and geometries, for example, do not.
Setting up a scene uses only 1 core. Hair and cloth simulations also only use 1 core.

If you consider moving up to a quad, also consider moving up to a 64 bit OS and installing more than 4 GB of RAM. Again, only useful for Poser 7 and higher.

The pen is mightier than the sword. But if you literally want to have some impact, use a typewriter

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