This page covers USB, ethernet, PCI, and other WiFi hardware that is compatible with OS X Macs. These WiFi adapters may also cost less than an AirPort Card, which tends to sell in the US$50-100 range nowadays.
802.11g will give you nearly five times as much bandwidth.
Older Macs may not have a slot for Apple’s AirPort Card – and even if they do, you may want higher throughput than 802.11b WiFi offers.