I'm not advertising for DLink or anything, but for about $10 or $15 bucks you can get a DWL122. It's USB based, but the great thing about it is, there haven't been any chipset revisions to it AFAIK. Only downside is that it's a B card. about 6 months ago when I checked, I didn't see any G cards that could go into promiscuous mode. the DWL 122 works like a dream when used with the Auditor LiveCD.

What I would do, is check the documentation of the specific tools you plan to use (i.e. Kismet, etc) and see what cards are listed in the documentation. Make a list of cards and then use the Manufacturers website and see if they provide links to Linux resources. Basically plan how big of a pain in the ass it's going to be.

Also check what's availabe and how big your budget is. The best cards were the hardest to find in the area and I hate getting stuff shipped to me.