People will hate me because of what I am going to say, but for $250 you could pick up a first gen MacBook Pro, which is much better for the types of applications you would want to run. If you really want a PowerBook, make sure you look out for later models, as these have superdrives, larger hd's, a higher RAM limit, better bluetooth and wifi, and sometimes a backlit keyboard. If I were you I would pass on anything that is higher than $150 at most for a PowerBook.
WAIT WAIT WAIT I was wrong. I used to have a 12" 1.0 GHZ REV B PowerBook G4, and I miss it dearly. However if you want to fool around with PowerPC Linux, I would recommend you get aquatinted with Mac OS X fully, and then try it. Once you do, there are many distributions of Linux for PPC. I would recommend just trying straight up Ubuntu, and seeing if that meets your needs.