Install the developer tools.
Apparently, they optimize your system.
It is a bug in the OSX installer that will be fixed in the update (or so I have heard), but the dev tools can fix it too.
------------------
G3 400, 384, 12 + 75 GB
PM 8500 - 150, 48, 1 GB
PM 7100 - 66, 32, 700 MB
PM 6100 - 66, 40, 4.5 GB