Wireless distance question

I have a question regarding the 24dBi 2.4GHz Wire Grid antenna. What kind of range does this antenna have when hooked up to a Linksys WRT54G wireless router? The reason I ask is, I don't have a DSL connection where I live but my neighbor down the road does, he lives 1.5miles from me. However there is a line of trees in the way down the road, and I can't get the antennas placed over the tree line, just slightly below the tree line. Just using 2 of these antennas and say 2 Linksys WRt54G wireless routers, can I make the link? Or do I need to add a power booster to each end? If so how much power and what is the FCC legal limit on this I have no idea?

Thanks,
Joey

This site should give you some answers to your question.
Radiolabs - Wifi Range Calculator

I have 2 longrange sections on my network.
8.9Km LOS with 24db grid array and a 14db panel at the other end.
2.1 non-LOS 24db grid arrays at each end.

All ends run WAP54G devices with a firmware hack to give 84mw output.

To take full advantage of your highgain antenna try to mount your AP at the antenna. Mine are mounted in a weatherproof box directly behind the grid array.

I hope this helps

Regards

Fred