I've seen online plans on modding linksys and cisco cards to accept external antenna connections. follow some of the links from the main netstumbler site, or do a search. the changes are true hacks, so c- for aesthetics. if that's not important, go for it.

if you want to save some time and cost, get yourself a used wavelan from e-bay. i scored almost all of my stumbling equipment for pennies on the dollar.