for anyone who needs to ask...
128 MB is a heaven for linux (depends, of course)
256 MB is more than enough for windows nt.
512 MB is enough for macintosh osx.
1024 GB is insufficient for windows longhorn. (ok, a joke...)
for anyone who knows that 512MB isn't enough - well, you know what you need.
if you're looking for a performance boost for your powerbook, you get it by letting certified apple personnell change the laptop's hard drive to some at least 5400rpm model. believe me, that will speed (overall) things up more than memory upgrade from 512MB to 1024MB.
and again - those of us who REALLY need more than 512MB know already how much is enough. for regular use, once the bare minimum spec (256MB for osx) is met, adding more memory does very little. going from 256 to 512 might speed some things, but for a 512 to 1024 upgrade one should really have some kind of a justified need to do it.
for a regular user like my father the only reason for a 512 to 1024 upgrade will be when someone asks how much memory his computer has it is easier to answer "one gigabyte" than answering "512 megabytes". and come on, regular joe would answer "i don't know" anyway, so no reason there at all
bottom line: 512MB is enough for osx. if you need more, you already know that.