Apple's Airport base station claims a maximum range of 150 feet. Other base stations have higher ranges. In any case, an antenna may be required on one or both ends. Fortunately there isn't too much in the way between the two structures.
Trees do unfortunately block that side of the windmill.
MacInTouch has an old story about a guy who successfully connected to his network from 8.5 miles away when in the line of sight. It's about SolarHost , a web hosting company that uses solar to power all it's servers. The Linux Router Project is a group that's crammed Linux onto a Floppy. Although the documentation is scarce, there appears to be a package for the Apache Web Server and the WaveLan wireless cards. I've also been spending time learning electronics fundamentals that I should have learned years ago.