I recently replaced my PC with a dual core AMD system with 4 gigs of PC4200 and dual 10,000 RPM hard drives. I saw a very slight increase in stability in P6's preview window and absolutely no difference in Firefly. From what's been said, Poser has a "choke point" where it simply won't use any more resources. There are a few tweaks here and there you can do, but honestly it's just picking gnat poop out of pepper. I'm sure somebody will be along with a more in depth explanation.
I know a lot of people here will avidly disagree with this, but instead of putting money into your system, you may want to consider a better rendering engine. Overnight? Damn! I have way too many deadlines to ever even consider using Poser to render again. Cinema 4D and Carrara 5 Pro aren't too expensive and they kick the hell out of Poser when it comes to rendering.
Just a thought...