One of my Linux boxen is primarily used as a server and frequently just sits around without monitor, keyboard or mouse attached. While previously, it used to be connected to the network via ethernet, now that I've got the USB wifi adapter for my machines, it's even niftier and only requires power to get up and running.
Today, I purchased and installed the Edimax EW-7316Ug wireless 802.11b/g USB Adapter. While I usually approach the purchase of hardware for Linux - especially for non-mainstream products - with a certain degree of trepidation, I was heartened to note during my pre-purchase exercise that the Edimax page specifically mentioned Linux compatibility and even provided a source code download. How rare is that?!
WPA 2 is pretty much the default encryption standard for wifi networks nowadays. However, standard XP installations do not support it by default. To enable WPA2 support, an update - KB893357 - needs to be downloaded and installed.
If you are not keen on going through the whole Genuine Windows (sic) crap, googling for KB893357 should provide you with a direct download link for the update.
The Linksys WMP54G, a wireless adapter that I bought after much (but apparently insufficient) research, is essentially not very Linux friendly after all. The card is detected by Linux as the following:
00:0c.0 Network controller: RaLink RT2561/RT61 802.11g PCI