Provided you can find the media for an OS X 10.5 Leopard Retail Install and you meet these specs:
Apple states the following basic Leopard system requirements, although, for some specific applications and actions (such as iChat backdrops) an Intel processor is required
51]
Processor must be any Intel, PowerPC G5 or G4 (867 MHz and faster)
DVD drive (for installation of the operating system)
At least 512 MB of RAM (additional RAM (1 GB) is recommended for development purposes)
At least 9 GB of disk space available.
Then yes, you can upgrade to OS X 10.5. This was the last release that supported the PPC architecture which your iBook is. However it will most likely slow the iBook down enough to wish you stayed with Tiger 10.4.
Leopard Info