Wireless communication over long distances
What sort of data rate do you need to maintain?

Long range communication is quite possible in open areas with very low power - a project I am currently working on incorporates data transmission over 500 KM using only 25 mW (miliwatts) of tx power - this of course relies on a line of sight, and data transmission is at only 50 baud. Even without line of sight, omnidirectional communications outdoors are quite easily achievable over the range you're looking for. 

As others have mentioned, matching your antennas with your intend usage patterns is important - can you give us more detail as to requirements/use patterns?