So I got this built in microphone

My iBook G4 (last year’s model) has a built in microphone (to my knowledge they all do)… Is there anyway in hell that I can connect an external microphone, say for a musical instrument? All I have USB, Firewire, and a headphone jack. This seems hopeless, but I have not given up on the USB ports; there must be some microphone USB converter, or maybe not… Anyone got any tips? Uh… external sound card… maybe…