To give you an idea...
I have a 4 year old Powerbook, 1.5ghz 15" screen, maxed out at 2gb of RAM
It runs the LATEST SOFTWARE from just about every vendor who matters and I have zero issues.
I know it's not as speedy as the new intel machines, but PPC processors were awesome IMO.
You would also be ill advised to not get a "pro level" computer or some sort if you are buying used that are of the older PPC architecture. The components used in the pro level machines are of a better quality and will last longer...I strongly believe this is the reason why my Powerbook still runs as good as it does...most people i know who have a Macbook of the same vintage have had significantly more problems than I have.
I have also been told by a guy who runs a local Mac shop that's been open for about 15 years that the components are of a higher quality and there's more attention to detail in the pro machines. So again, adding up to a longer lasting machine.
You can score a used Powerbook for 400.00-650.00... then again, save up another 4-500 and you can buy a new macbook that will kick the ass off a Powerbook in terms of speed.
my .02.