Go for the fastest processor you can get. Poser 4 is the most intense user of CPU power I have seen. I am running an AMD XP2000 with 1.5 gigs of EEC Registered Ram (OS win 2k pro sp2). I also run a core temp monitoring program.

For instance Graphics apps like Paint Shop Pro with files of over a gig in size open run the core temp at around 37 Celsius. If I turn on Poser 4 and load a millenium figure the temp rises to 41/42 C. I have 9 high speed fans and very good air flow/controll but Poser still pushes the core temp around 4 to 5 degrees above any other program including modelling programs with very large models open. Task Manager shows that poser just pumps the CPU like no other program.
More Ram can help but Cpu speed is the crucial element.
I am still waiting on Poser 5 but I suspect if anything it will push the CPU even more.
Regards
STORM