10.4 on 800 mhz G4
I have one of those and on this machine I did upgrades as soon as they came out (not mission critical machine). No problems, until Tiger. Typically I would upgrade OS rather then do clean install or AI install, just to see how will it go. Tiger gave me kernel panics at restart and startup, so I did AI install after that.
There was another interesting issue with Mail, ical and Address Book apps after itunes upgrade (roughly same time frame). All of new versions ended up in archive folder and the old ones in App. Folder, which caused some problems (won’t go into details), but after I figured this out it was easy fix.
Overall there are speed improvements from earlier OS X versions, mainly finder and safari.
If it’s not too much trouble for you I would recommend clean install to start fresh and you’ll be one happy camper. Also I would recommend to max-out RAM if possible.