I think memory is much more inportant than raw procesing power.

Bearing that in mind, your OS will limit available memory as mentioned above, as will the verson of Poser you use. PoserPro 2010 seems to handle memory useage much better than previous versions.

I also run PoserPro 2010 on Win7 64 with 12gb RAM, and have never again seen "unable to render" or "out of memory" no matter what I throw at it.