I've done some WLan on Slack9.1 and out of the box I can recommend the Netgear card it works great directly. I've used a Xircom card as well But I don't remeber the name right now and that worked as well OK. As a rule most 802.11b card will work great out of the box, but the + cards must have a new driver installed.
I've used a D-Link DWL-AB650 and it worked great with the madwifi driver, BUT the PCI card (DWL-AB520) is a big pain in the a**e. It's instable and a mess to get to work proper. I might have mixed thoose names around but one is PCMCIA and the other is PCI card.
I would stay away from any card that's not supported by the kernel out of the box, just to much hazzel to get it to work...