It runs smoothly and quickly for me on my dual 1Ghz G4; unless I've got a large and complicated scene. A big scene can easily take a couple gigabytes of virtual memory. If you don't have enough physical memory, Poser can spend all its time reading and writing your RAM pages to disk. That's virtual memory.

Even in high-resolution Firefly rendering, Poser doesn't take an entire processor; it's too busy waiting to read and write virtual memory pages in the swap file. In general, adding RAM is more likely to help Poser than faster processors. (At least, until you've added enough RAM to keep it happy, adding processor power won't help much.)