Suggestions for setup? Antenna Question

Mainly I'm wondering what dBi an antenna has to be in order to give a good omni directional range to a very open area (with no major interferance). Ideally the range would go .5 to 1 mile omni directional. Or .5 to 1 mile in one direction and about 300 feet in the other direction. a couple more details below:

I have a D-Link 4300 wireless router and am looking into purchasing a larger antenna to mount on the outside of the building. we have several buildings within half to a full mile in different directions of the router, its all very open, no major obstacles like trees or anything. though some of the buildings are much higher in elevation, because they are on a large hill. Our closest neighbors are at least 3 miles away on the other side of pretty much a mountain... so theres no risk of interferance really.

Basically I don't know anything about antennas, and am looking for an antenna that's compatible with our router, but has a good range for what i described above.

any suggestions?

thanks!
max

Ask FredWeston:

See Fred's post...