Just so you know: if you want to use the Apple Pro Speakers with your PowerBook, get this:
http://www.griffintechnology.com/products/ifire/ . It plugs into your FireWire port and gives you an Apple Pro Speaker slot.
Why do you want speakers in particular? If you really want speakers, go with Hardon Karman or Altec Lansing, those are the best. But I seriously recommend headphones instead, if you're only going to be listening by yourself. Get yourself a nice pair of Sennheiser HD 570's ($130) and you'll notice a huge improvement over pretty much any computer speaker you can get, period. These things are absolutely amazing, I have a pair and they're so crisp, with the base punchy but not overly boomy and the highs so clear...
But I'm starting to sound like some raving fanboy, so I'll shut up now. My point: Ask yourself why you want speakers, if you're only going to listen to music by yourself headphones have much better value and sound quality. If you're going to listen to music with other people all the time though, by all means get some good Altec Lansings or Hardon Karman.
Good luck on your quest for good sound!