when i got my new 466mhz G4 I knew that it didn't have any kind of analogue in (it has audio out (analogue) & a digital audio out)....
... so i brought an iMic (
http://www.griffintechnology.com/audio/imic_main.html)
it's a USB mic... it has Audio In/Out, & works ok under MacOSX (aswell as 9).
the nice thing is that there is no need to install any software... it is all in the device itself....
.. however, i have found a few 'problems' under X
1. when u plug it in, it disables the Analogue audio out, so u have to use the analogue out on the iMic (which is not a problem)
2. It doesn't appear to support passthrough...... this is more of a problem... but i've not checked this out properly yet....
ps: iMic cost me around £40
pps: [i think] the sound quality is better through it...