Creating wireless AP

[epod69]

I am hoping someone has some experience with creating a large wireless mesh or city-wide wireless. Being that the good commercial Access Points are in the several thousand, we at work are wanting to build our own access point and have Linux on it that we can configure and manage it with. I can get a industrial board designed to be used as an access point with cpu and memory on it for less then 200 bucks. With the daughter card it would have 4 mini-pci slots, which you can use to put the mini-pci wireless adapter card in.
Here is the product site of the mini-pci wireless adapters which have pretty powerful radios:

http://www.ubnt.com/super_range.php4

I was also then looking at a outdoor enclosure that has a fan and heat as well. I was then going to mount it on TowerMast:

http://www.antennamast.com/products/prodSysTowerM.htm

, and then mount a high powered omni-directional antenna on it as well.

The first unit I want to build I would put at home and test it out. I would only use one omni-directional antenna then. For what we are wanting to do at work, we are wanting to have three antennas. Two directional site to site antennas which we want to use to connect to other access points 20 miles away and then one omni-directional antenna to service everyone around it.

So here is my question then, are we crazy thinking we can sell internet at broadband speed using only a "single radio". I have heard of using a "dual radio" in which that would not suffer from as much traffic.

Anyone have experience in setting up this large of a wireless network?

Thanks all for the help!
Jesse