I've just installed an airportcard in my PowerMac G4 (QuickSilver 2002) and an airport extreme card in my iBook G4.
The PowerMac is sharing it's internet connection via the airportcard in the study and I'm sitting here on the couch without any wires at all posting this.
The installation process was fairly easy once I figured out that to insert the card in the iBook required a little force (it has to "click" so the metal braces holding it in place actually fit).
The PowerMac was also a bit of a puzzle. Turned out that while the airport extreme card in the iBook was to be inserted with the words airport extreme visible, the PowerMac's card was to be inserted upside-down so the word airport wasn't visible.
Once I figured out those things everything went smoothly and actually creating an ad-hock network and joining it was the matter of less than a minute.
Joy of joys, now I can aspire to become a real couch-potato.