I got an iBook with the standard 30 GB drive too, which I'm starting to regret. But I'm thinking of upgrading in about a year, when the warranty expires anyway. I'm pretty confident I can do this myself. It's not that difficult. I find Apple books to be robust not only on the outside, but on the inside too. Things would be different when it would have been an intel laptop though, they can be a PITA to open up.
You say you want an internal expansion. The reason why you would want that is obvious, but you may reconsider an external drive nevertheless. Firewire drives are cheap and my FW drive is way faster than the internal one, so speed should not be the reason. I keep most stuff on the internal drive, but copy large files (movies, mp3's, backups) to the external one. It's not ideal, but I'm happy with it.