In my experience, the iSight is the best webcam out there for any firewire Mac. It's small, light and stable, has good picture quality and all the features you need (good clips, closing shutter, LED indicator, etc.).
It's tempting to save a few bucks and get something that 'might' be as good as the iSight, but then, you could also get a cheaper laptop and 'hope' that it's as good as the iBook.