Newbie question: iBook upgrades?

[Jean-Paul Thuot]

Hello all,

I am completely new to Apple (if you discount computer science in grade 
8 with apple ][e's) and have just this week purchased a G3 600 12.1" 
iBook (dual usb).

I understand that I can (and must) upgrade the RAM from 128, but I'm 
wondering what else is upgradable in this system.  HD, CD-ROM, CPU?

Any advice for this humble newbie would be gratefully appreciated, on 
this topic or any other pertaining to this model of iBook.

Thanks,

Jean-Paul