Concur, but you don't need a lot of power to get wifi distance. With line of site, we got a 20 mile link running between a linksys AP and an orinoco PCMCIA card, each running about an unaltered 28mw, using 24dB gain antennas. We use this stuff to supply internet access to rest stops at a local bike ride, and never use power amps. We've done a reliable 8 mi link for the past several years this way.

To the OP, if you are really serious about this, pick up a copy of the ARRL handbook and some books on antenna theory from the ARRL store:

ARRL Store

And consider getting your ham license, as you can get seriously illegal very quickly experimenting with this stuff without one.