With directional or parabolic (dish) antennae you can cover over 1000 ft, with a point to point connection. You need to be very exact with the placement and you need a second AP and idem antenna to receive the signal. Placing a third AP in the middle will help but you need some more antennae.
Antenna stuff: http://setup-wireless.blogspot.com/2008/11/wireless-antenna-types.html
1000ft experiences: http://www.wifi-forum.com/wf/showthread.php?t=81226
Good luck,