Although there are many different wireless cards made by many different vendors available for purchase today, there are only a small number of companies that make wireless chipsets. For example almost all 802.11b cards in existence today have either an Intersil Prism or Agere Hermes chipset in them. Chipsets are important because usually wireless drivers work with Chipsets rather than individual cards. Finding out whether your chipset is supported in Linux is the first step in getting your wireless card working.
You may be able to find out what chipset your card contains by looking at one of the following lists.
http://www.linux-wlan.org/docs/wlan_adapters.html.gz
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An excellent question, it depends on what you want to do.
The best cards for general 802.11b use are the Proxim Gold cards which contain a HermesWirelessChipset, these are well supported by Linux via a number of drivers. They will work in both AdHoc and Managed modes but they cannot do HostAP. If you want a card that can do HostAP (act as an AccessPoint), then you need to buy a card with a PrismWirelessChipset.
DickSmith's sell a CardBus wireless card (product number XH8225) that uses the Atheros chipset. These can be made to work under linux, but it is fiddly (see below) -- it doesn't come with the required linux drivers, and their website has a snapshot of the driver from CVS but it is too old to compile against kernel 2.6.9. Unless you are comfortable compiling a kernel from source, maybe you shouldn't consider buying one of these cards.
Alternatively, you could run Ubuntu, and install linux-restricted-modules, and bingo, instant support. The MadWIFI project provides distro agnostic support, also. -- GreigMcGill
The following is from LinuxKernel 2.6.9, but should be similar for similarly-versioned numbers
Check you have support (modules or built-in) for the following:
Part of CategoryWireless