Forget the 802.11b.
Buy an 802.11g unit; they're backwards-compatible, and you'll have the full 54Mbit when and if you get a new machine later on.
I really like the idea of the Airport Express, though my D-Link DI-624 (you can get them for about $75) is flawless, once the proprietary special Xtreme G mode is switched off.
-s*