As Royal Pain stated earlier, depending on the model of laptop, changing the HD will be either really easy, or a real pain in the ass. I changed the HD in my G3 ibook once, and I think I had to unscrew about 25 or so screws just to access the HD. Laptop was going back to Apple to get the screen fixed, but I had already broke the HD, so I had to replace the HD myself before sending it to APple, or else I would have had to pay to get the video fixed.
I imagine the G4 ibooks are the same pain in the ass to upgrade. The Macbook designs made it much easier to work with.