Douj,

(1) about free space
My personal rule is to let free, at least, 2 times the space used by the installed OS
Panther, if you do not have suppress languages and printers drivers, will use nearly 2.5G.
And I give to my friends the following easy-to-memorize advice (for Panther):always let 4-5G free.

(2) about cleaning cache,...
CacheOut X

(3) manually running the Apple maintenance scripts (if your mac is not on between 3AM - 5AM):
Macjanitor

(4) about Tiger; Tiger is not a free update, you'll have to buy it
Your Panther 10.3.9 is a good OS for your G4 and I'm not sure you'll need to go to Tiger: my daugther runs Panther on my old G3 and runs LW fine.
The shared folder is located to an external FW drives (as its Itunes Library) to avoid the G3 internal hard drive to run out space

So in short
* always let 5G free
* put the big files (movies, Itunes library, ...) on your external drives
* runs Macjanitor on a daily base to ensure best performances of your machine

For any problem, questions: The Bible of Dr Smoke

A remark: use CacheOut to clean you browser caches and other normal caches. Do not use CacheOut to clean you system caches on a daily base because this specific feature of CacheOut (which requires the admin password) is a hard procedure including a reboot: only use this extreme feature if you face big boot-sleep problems

So, with all this, I hope you G4 is ready for a new life

Phil