I would want a video card which allows me to preview in Poser using OpenGL with hardware shading enabled.  Nowadays that can be done with most mid-range cards.  If you also use Vue, a goodly amount of memory on the video card will allow for a clearer preview.

     It won't affect Firefly render speed.

     For render speed, you want processor cores.  I would consider an i7 HyperThreaded quad-core (eight render threads).  If budget allows, then the H/T hex-core Xeon, or the H/T octal Xeon.

     Have a generous budget?  Get a motherboard with dual processors.  Two i7 quad-cores would render sixteen threads.  Have an absurd budget? Get dual hex processors (expensive!) or max out with a dual octal (very expensive!  but it gives 32 rendering threads).

     The other factor to consider is memory.  It doesn't affect render speed...unless Firefly approaches the limit, in which case it slows down terribly (and thrashes your hard drive).  So you want enough memory that Firefly won't run out.  How much that will be depends on the scale and complexity of your rendering habits.  Try for at least 12GB (triple channel) or 16GB (dual channel). 

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