Most of the WIFI card will plug and play. If your are running Gusty the newest
version of Ubie. Here is a link for cards that run and go. Some do need some
mojo to run. Like DL the drivers and ect..
https://help.ubuntu.com/community/WifiDocs/WirelessCardsSupported
What you could do is take the live Ubie CD to the computer store and see which
card works.

Do your research before you buy. Wireless card vendors change chipsets
without notice or obvious version rev's.

Stay away from the Realtek 818x based cards. The driver is broke in Gutsy!
https://bugs.launchpad.net/ubuntu/+source/linux-source-2.6.20/+bug/78255

In general, I recommend cards with the atheros chipset.

Note: My comments are related to wireless cards in general not to USB
specific devices.

I'm using the very common BELKIN F5D7050 usb 801.11G ADAPTER
version 'V4000uk' using a Realtek driver included in Ubuntu Feisty &
Gutsy. It works transparently. I have another ver 2000UK which does
not use the same driver and is not Realtek so I have not experimented
with it. (Let sleeping dogs lie!)

I also use a much older F5d6050 8011b on Mint and it also works out of
the box on Mint 3. You'll get one of these for a song on ebay.