I don't know Your working environment (walls, distances etc), but I really happy with my D-Link DWA-520 (Atheros AR2414 chipset). It works with madwifi (current svn brunch) like a charm through 3 walls and about 10 meters of the distance. This card is regularly lost its connection with ath5k driver until I switched to the madwifi.
My previous card was D-Link DWA-510 (Ralink RT2561 chipset) and I didn't succeeded with it. It also regularly lost its connection and speed was too sloooow! I sold it and bought DWA-520.
I suggest You to buy any card on the atheros chipset. List of devices can be found here: http://linuxwireless.org/en/users/Drivers/ath5k
I use this card on the slackware distro with the vanilla kernel 2.6.32.x
P.S. Sorry for bad english. It isn't my mother language.