Newest OS you can put on that is Leopard 10.5.8 but brace yourself, it's not cheap.
As it stands right now, with Tiger (you should update it to 10.4.11) you can take iTunes to 9.2.1 and no more.
Use the full installers, don't go via the Software update.
Start with QuickTime for Tiger: (do this first to make sure the QT engine is properly installed, yes, iTunes uses it)
http://support.apple.com/downloads/QuickTime_7_6_for_Tiger - then get the LAST version of iTunes for Tiger.
As to what iTunes 9.2.1 supports as far as iPods and the like, I haven't a clue.