I'm upgrading to the AMD r9 270x ($200).  GPU choice has no affect on Firefly rendering as Firefly is CPU based, though a fast modern GPU will give you much better OpenGL previews (with hardware shading enabled).  The GPU is really only an important consideration if you plan to use Octane or Luxrender/Reality3 (and again, Poser's own OpenGL preview) which do use the GPU.  For third party GPU based renderers that are CUDA based, an nVidia card is a must.  For OpenCL renderers (such as Luxrender/Reality3) I would instead suggest an AMD (ATI) card.

OpenGL= Open Graphics Library

OpenCL= Open Computing Language

Luxrender is a renderer, Reality3 is a Poser plug-in to bridge Poser and Luxrender.