I have a old, really old, iBook Dual USB. It has never given me any problems and with a replacement battery is still a quite useful machine - if you are not in a real hurry.
The machine did not come with wireless, I went wireless after the AirPort card stopped being made so used a USB stick from Belkin with the Ralink drivers for MacOS X 10.3.
However knowing that Apple would eventually stop supporting that version and given the age of the machine I would have to chose an alternative OS at some point I chose to use Debian Etch on it.
Problem was the USB stick has never worked with Etch, I purchased an EdiMax stick that was sold as Linux compatible but this required drivers to be compiled for it - these drivers did not like the PowerPC architecture at all so rather than remain isolated I investigated alternatives.
Luckily I came across the ASUS WL-330gE, this attaches to the Ethernet port and once configured can act as a Ethernet to wireless gateway.
The unit attaches connects to the Ethernet port and is powered from either the USB connector or a separate power pack. Conveniently I have a USB connector adjacent to the Ethernet port so I plugged it in set my IP address to one within the adapter's default range and configured it to match my wireless network.
Once I reconfigured my Ethernet settings to use DHCP my machine connected via the adapter to the wireless network.
If you are a complete Linux novice I would strongly suggest that a machine with Linux pre-installed with drivers for wireless would be a better match for you.
People who know a bit about Linux and know how to reconfigure network adapter settings and just want their machine to connect via wireless may want to investigate the ASUS unit.