One way they distinguish between the lines is that only the professional models support monitor spanning, where the operating system desktop can extend to more than one display screen, so you can drag windows from one to the other and have different programs on different displays, for instance. I use such a setup on my older Power Mac, and find it very handy. The consumer models let you use an external screen, but only to duplicate what the built-in display shows. However, those models contain graphics chips that are perfectly capable of monitor spanning -- Apple has simply disabled them from actually supporting it.
Now some enterprising Mac enthusiasts have managed to trick some iBook models into supporting monitor spanning. As usually happens, hackers found a way around arbitrary restrictions eventually.