The question of how much RAM is enough for Poser is dependent on your usage habits and -to a lesser degree- how many cores you're feeding.  The relationship is not linear;  i.e., double the cores will not use double the RAM, but a HyperThreaded quad will use more RAM than a dual core during rendering.

If you create complex scenes, use sophisticated materials, render with raytracing/IDL/SSS enabled, and render at large pixel dimensions, then the 12 or 16 GB option may well be your best choice.  If your Poser habits are not so grandiose, then 8 GB should be plenty.

For the hard drive, you might consider paying more to get a larger buffer cache for a given size/speed hard drive.  That would help the HD keep up with the audio recording data flow.  An SSD for the audio work would be a good option.

Poser 12, in feet.  

OSes:  Win7Prox64, Win7Ultx64

Silo Pro 2.5.6 64bit, Vue Infinite 2014.7, Genetica 4.0 Studio, UV Mapper Pro, UV Layout Pro, PhotoImpact X3, GIF Animator 5