When we bought our iBook G4, we purchased the (external) LaCie d2 Dual DVD+/-RW that was available through Apple's website (I believe it was between $200-$250). The main reason we chose this model was because we also have a PC, and this external burner is compatible with both PC and Mac. So far I'm really happy with it. LaCie has their own website if you want to check it out for more info on the product... It also comes with Toast 6 Titanium software.

PS-One thing you'll want to know....so far as I understand, external DVD burners are not compatible with iDVD--at least, they weren't when we purchased our iBook in December 2003. We used iMovie to make our movies and used Toast 6 Titanium to burn them to DVD.