Buy a card that uses an Atheros chipset. Most (if not all) of them are known to work with the Madwifi driver. See
www.madwifi.org
I bought myself a D-Link DWL-G650 recently and it works perfectly with Ubuntu Breezy (incl. monitoring mode)
A few weeks ago in a shop I saw one card that had a "works with Linux" sticker on the box. I almost bought that one just because they bothered to support Linux but then it cost about twice as much as the DWL-G650.
stimpy