This is no official word, but as far as I can see it, 500MHz and 256 MB should be sufficient to run Poser 5. It will probably slow down in extremely "hairy" situations. ;-)
Do yourself a favour, don't worry about the Poser 5 requirements now. You will mostly get unbased speculation instead of facts anyway. Let's talk about Poser 5 when it's out, OK?
"the more memory the better" is a universal rule of thumb and applies for almost every program out there (except Win98 that doesn't know how to handle 1 GB RAM), not only Poser.

Stefan

As always, everything written above is purely my personal opnion.