Since you mentioned "traveling" in the subject line, I say forget speakers and get a good set of headphones. You can use those on an airline, in the car, in a waiting room, or where-ever without danger of being pummeled by those around you and you won't need extra power supplies, etc. I have a pair of Bose headphones, not the noise canceling variety, that are lightweight, do a good job of shutting out external noise, and have excellent audio response. The noise canceling headsets are nice but way too pricey for my pocket and in spite of the commercials I did not find the ones I tried that much more effective than regular headphones with a good ear covering cushion until I got up into the $400+ range.
If you take traveling out of the picture there are any number of good speaker systems you can use with your PowerBook. Many of them weigh more than the PowerBook and most are considerably bulkier and require external power to run them. But on a desktop, who cares?