I have a Dual PowerMac G4; it's from before the time of AirPort Extreme Card slots. It does has an AirPort slot, but only for wireless-b AirPort cards ... which you can no longer get from Apple, and other place charge a premium. Macwireless.com does have WiFi-G card that work for this unit, but once again, they charge a premium ($150 was the cheapest I found).
But you can use a cheap Motorola card, WPC1810G, which I found for 35$ at a local Radio Shack on clearance. I found that this card was confirmed to work based off a website I found using google.com. Unfortunately, I lost the reference to that site; sorry. Sadly, this card stops working once you upgrade Tiger to 10.4.2. There is an update to the AirPort kernel extensions, from version 3.5 to 4.0. I don't know if Apple did it on purpose, or just doesn't care to make sure it doesn't break third-party wireless cards. But this update does.
Since this update is mandatory if you want all the other fixes, and you can't choose not to install just the AirPort update, you can't roll-back (easily). So I personally had reinstall Tiger from scratch to undo the changes.
But this time, I backed up the following files.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20051011073837294