For my money, a Prism3 card is the way to go if you'll use 802.11b. The original ORiNOCO cards are equally supported, but slightly underpowered. The SeattleWireless H/W Comparison
lists most of the available and retired client cards along with details about their innards. You can get a ZyAIR B-100 (80mw Prism3) for about $30. It's a re-badged Zcomax, IIRC. If you will use 802.11a, g or a combo card, the drivers will be more bleeding edge.