If you don't really need the connection to be wireless, it's a lot less expensive to go with a regular router/firewall. In Canadian dollars, about $150 for a router from LinkSys, plus maybe $40 worth of ethernet cable, vs. $600 for the Airport base station plus about $60 per card, last time I checked.
Typically these boxes will have a four port ethernet switch, plus a basic firewall. The speed you get on the internal network would be about 40-50 times faster than Airport.
Wireless networks also are notorious for bad security - there is a practice of hacking into corporate networks by sitting in a park across the street with a laptop and Airport card...