JAWUG

For 2.4Ghz links over 5km's, I would recommend going for a Grid Antenna.

They are highly directional, which means they dont pick up too much noise from their surroundings. Their beamwidth is very focused, so you may have to spend some time aligning them properly. But once they're in place, you wont have any problems, and you'll definately be happy with the results.

Uniterm and Miro sell Grids.

2.4Ghz 24dBi+ Grid Antenna:

:-)