You want wireless NICs to boost the signal?
If you want to create a small ISP over a wide area, then 802.11a/b/g is not the solution. g/b operate at 2.4GHz and I believe 802.11a operates at an even higher frequency. Those just won't do at all when you are expecting to go through trees and walls. You need to use some kind of No-Line-Of-Site broadband technology. When I was in college, we were playing around with exactly that. We used products from WaveRider (
http://www.waverider.com/) and it worked for miles around. This is what some wireless ISPs also use here in the US.