My iMac G4 800 MHz Flatpanel does not have an original (and now practically unobtainable) AirPort card. Even if you find one for 100 euro or more, and it turtns out not to be a scam, you still only have 802.11b, not g, wireless. That is a problem! Here's the fix. I purchased a Belkin Wireless G Gaming adapter (F5D7330). The adapter is hooked up to the UTP ethernet port and it draws power from a USB cable (of which it has two, but one is enough). So you do not need the power adapter, and there are no net cables involved.
This has allowed me to connect to my AirPort Express router. It feels great making an 802.11g connection with a machine that is deemed not AirPort Extreme Ready.
Of course, I want to secure my network. I can set up my Base Station using the AirPort Assistant, but how do I setup the adapter correspondingly? AirPort software only works with the original unobtainable AirPort card. After I tried everything else, I read the manual. Go to System Preferences » Network » Configure and switch to manual IPv4 (or set it up as a new location). Type 192.168.2.220 for the IP address, etc. Follow the instructions in the manual which are intended for Windows. Use the reset buttons on your Base Station and adapter in case things do not work out; even unplugging the USB may be required. Success!
[robg adds: Note that this is not really a portable solution in terms of size; it's more like a typical wireless router, not a cardbus solution.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2007110507224365