The requirements for the iPhone 3G are Mac OS X v10.4.10 or later and iTunes 7.7 or later (necronym is incorrect - 10.5 Leopard is
not required - I am on 10.4.11 and iTunes 8, personally).
You
cannot run under 10.3, so you will need to load you music on another computer with at least 10.4.10, or upgrade your G4. Even though Tiger is no longer available from the Apple Online Store, if you call Apple Phone Sales they will sell you the Tiger upgrade (same pricing as Leopard, though, so if your G4 meets the
minimum specs for Leopard, that would work too).