Mac Mini G4 Airport upgrade?

The G4 minis that didn't have Airport already installed lack more than just the Airport card. To add wifi interrnally you'll need the Airport card, plus a small adapter board (often called the 'mezzanine card') and the airport antenna. The entire kit was available as a service part via approved service providers at one time, but is now discontinued. However, it can still be found from time to time if you look.

The part number is M9870z/A and includes Airport and Bluetooth. It's currently listed at http://www.welovemacs.com/m9870za.html but this looks to be double the usual price it has been seen at. http://www.mac-pro.com/s.nl/it.A/id.1501/.f?sc=2&category=22 lists it at the usual price. In neither case can it be assumed they are in stock without checking since these are pretty rare these days.

If you get one of these it's not hard to fit yourself, or a local approved service provider will certainly be able to do it for you - for a charge of course - probably around 1 hour labor time.

Alternatively, you could add wifi externally by adding a USB dongle or wifi bridge wired to the ethernet port. USB is typically not a good way to do it where consistent data throughput is needed, but an external wifi bridge would be cheaper than the internal upgrade kit. The downside is that you would not have as easy control of wifi as with an internal card, though signal level might be a little better depending on your specific circumstance.

Citizen Spain,

Some routers can be configured to serve as a wireless ethernet bridge. Check the documentation for your router to see if it has that feature/option.

For $30 that device may be worth trying, but as I said, the problem is that even if it works as hoped, USB is a poor medium for networking. It's downfall is that data rates are dependent on overall system activity, so if you're doing something fairly system intensive and trying to use the wifi connection at the same time for a task involving a data stream, you're going to risk finding dropouts, disconnections and poor network performance. That doesn't mean it won't work, but that there is a risk that it won't work efficiently at all times, which typically network connections are needed to do.

The external, ethernet wired bridge is the better technical route to go, and as Boece says, many routers can actually be put into bridge mode to accomplish that. The cost would be a little higher though - there's a Belkin model at $39, but as Boece can attest (if I recall, Boece?) it's not easy to get into bridge mode with a range of non-Belkin products!

I had the same deal. I upgraded my old g4 mini with the upgrade kit mentioned earlier from PROVANTAGE.com. I got a copy of dissassembly instructions from the web. (don't remember where). Took all of 30 minutes and works like a charm.

Not sure if you fixed this issue, but you could always obtain an Apple Extreme Base station either the new 'N' spec direct from Apple or the earlier 'G' spec via e-bay or similar. Use an ethernet cable to connect up your Mac Mini to the AEBS.
You may need to check on compatibility with your existing wifi network.My Mac Mini (original spec-no Airport card fitted) is connected to my AEBS which in turn acts as a remote base station via WDS-works perfectly!
This link may assist (page 15)
http://manuals.info.apple.com/en/AirPortExtreme_Base_Station_Setup_Guidev4.2.pdf
Alternatively, why not just hook up your Mac Mini to the ethernet port on your exisitng router?