Going the way of USB might not be the best as that machine only has USB 1.1, and therefore will be very slow.
If you're only looking to make it wireless, simply pick up an Apple Airport Card. iMac G4's used both the original apple airport card, and airport extreme. What is your processor speed?
Yup, that is a G4. But also knowing some more info would he helpful, as some of these iMacs used the original Apple Airport Card, and some used Airport Extreme.
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