I am running 10.5.8 on my iBook G4 1.33Ghz 12". That was the last 12" iBook they made before the Macbook came out. I have 1GB RAM but want to upgrade to 1.5GB. Mine has a 512MB soldered on the logic board and a slot for another 1GB. With just 1GB it's fine for most usages but that extra 512MB might help for a few things. It's useable for sure for what I do on my iBook.
Max out your RAM and it will probably be fine.