*This is a operating system thing. As far as I know all applications that run under OSX use the second processor.*That is not the case. MacOS X will be able to let any application as a whole run on the second processor, but not to divide its workload on multiple CPUs.Unless an application is explicitly written for using multiple threads it will not notably gain performance from a second CPU. Wether an application is able to use multiple CPUs depends only on the application itself and not the OS. Even with "modern" OSs like XP or OS X, making an application multithreaded is not an easy task.

Poser 4 does not use multiple threads for rendering (that's about the major point for SMP optimizations in a 3D app). I will not comment on Poser 5 as I know you will quote me on that and use everything I say against me. Sorry.

However, a second CPU will help you when running multiple applications as in my experience my system is a lot more responsive with two CPUs when performing CPU heavy tasks like rendering or compiling. If you plan to do some compositing/editing/encoding while rendering or using Poser and one of the plugin-enabled modeling/rendering suites (Lightwave, Cinema etc), a second CPU will give you additional speed.

That said, I am typing this on a dual Celeron 533 which I am mostly happy with.