If you can get to a window in each building so they can see each other bridging this distance is easy. Just get two access points, I use the wap54g models from linksys. Set them up for bridge mode. Also go to
www.freeantennas.com and construct parabolic reflectors out of file folder stock, aluminum foil and scotch tape using their templates which you print out on a printer. If you use dual antenna access points then make two for each one. These reflectors slip on the existing antennas. This will give a 20 db increase in signal (two 10 db reflectors) so that you have, in effect, a 100 fold power increase of signal strength, so you should wind up with good throughput. You will, of course, have to run some cat5 cable from each access point to a switch that the computers in each building are connected to. Also you will need to concern yourself with security issues (changing the encryption key regularly, using netbui for file sharing, and such. You probably would want to change to the newest boxes that have even more enhanced security when they become available, but for now, the setup I described should work in that it will easily cover the distance without having to go outdoors and mount antennas, have expensive low loss feedlines, etc.