If you want an external antenna, you should look at some of the PCMCIA solutions. I looked briefly for a solution to use with an Airport Card and couldn't find one (course, that may be cause I looked briefly)
I know most, if not all, of the Orinoco 802.11b cards will accept an external antenna. Honestly never looked at the 802.11g cards around for plugging in an external antenna.
As far as a router "seeing" your Airport Card, it depends on who is running the router. Every router can "see" your Airport Card and your connection. Otherwise, you wouldn't be able to connect to the router.
Honestly, if the wireless router is open for you to find and connect, they probably won't even know you're there.
Just in case, make sure your default settings are good to go. Go to System Preferences/Sharing and take a look. First button is "Services" which should all be turned off unless you have a need for them. Second is "Firewall" which should be on and nothing checked as "on" unless, again you need it.
Otherwise, wardrive/walk/sit to your heart's content
