Well, most anybody would say that Adobe's Creative Suite 3 is the best overall Photoshop programme. My only concern would be whether or not this application would run natively on a Power PC processor such as the one on your iBook.
Do you plan on doing a lot of intense photo editing? I know that Adobe makes Photoshop Elements (99$) that runs native on a PPC processor. That'd definitely be something to look into if you're not 100 percent sold on CS3, which is rather expensive. (399+USD) Also, I think that Aperture (Apple's higher end editing software) is brilliant.
There are a few options. Describe to us what you'd most like to do with the software, and we'll hopefully be able to help you out a bit more.
Edit: Just noticed that your iBook is rated at 600mhz. How much ram do you have? Have you tried iPhoto? If so, what do you think?