You can save a total of maybe 300 to 500 MB of disk space by removing OS 9 (aka Classic).
However, if I were you, I'd invest in a new HD for your old iBook. I'm currently between 'new' computers, and have been forced to run on an old PowerBook 3400 until Apple comes out with their new 15" PB. However, going into this I know that the old 2GB HD in the 3400 wouldn't be enough. So I spent about $100 and bought a new 40GB HD. Now, this computer has more disk space than all previous computers I've owned, combined (including this 3400 before its upgrade).
Larger HDs are cheap, even for laptops, and not that difficult to install. If installing one scares you, find a friend to do it, or spring for the $50 that a computer store will charge you to do it.