"Size doesn't matter" or so we're told.  I do animations using P3 on both a 1.2 Ghz PC, and a iMac G3 (450 Mhz).  I use P3 because it fits within my personal styling aesthete, and even my clients like the lo-tech look (I tend to do anims in Toon mode).  I also use P6 on a dual chunk G4.  Animating on this takes a whole whack of time.  I loaded P6 on a new intel based dual chunk Mac as a test and animation times (full render P6, Jessi dancing)  still took a lot of time for a 30 second clip.  the more info you are asking the computer to handle, the more time it will take.  I started this Poser addiction with P3 on a Mac Performa 6400 (running a screaming 180 Mhz with 96 Mb of Ram) and still turned out work.  So, as a matter of curiosity, what kind of work are you doing?