I think it's more important to fully understand the Firefly renderer before investing in new hardware. Read up on how the renderer in Poser works. Also, be sure not to let the renderer work with things that doesn't matter in your picture, for example using texture maps that are larger than the final render. Check out the info at the e-frontier homepage (for example, this: http://www.e-frontier.com/article/articleview/1406/1/323/, same as Chapter 7 in the Poser Tutorial Manual) and read the Poser User's Guide Part 7. More RAM might help when doing large renders (lots of large textures etc), but in my experience the settings of the renderer are far more important.

www.oscillator.se/3d - Shade tutorials and Poser freebies.