I agree with what's already been said. Before it died of the rather infamous video chip solder joint
issue (and incidentally P, since you asked before, recently came across
some info that this issue affected ALL iBook models) I had 10.5 on it,
and it just wasn't that great of a performer. IIRC, this was a 1.4GHz G4
with 768MB of RAM, so it should have exceeded the Leopard min specs
quite handily, but performance really was at the barely acceptable
level.
If this is an iBook G3 model, there's no real point in bothering to do
anything with it. It's just too old to even function as a decent web
browsing machine. Same with all but the last of the G4 models, and if
yours hasn't died of the cold solder joint on the video chip issue
already, it's probably living on borrowed time. It's probably not worth
sinking any amount of money into this system.
But if you're in a will-not-be-denied kind of mood, only the retail copy
of 10.5 will install on any system. The restore discs that come with a
system are locked to that specific model.