If it were ONLY that simple!
First - you will need a whole lot more than a consumer grade cheap antenna!
To cover 2km with the users able to use the adapters that are in the machines they currently have, you will need several integrated access points around the area. There are several "Mesh" systems that allow this. Each AP will need to me mounted so that it is above the roof tops and able to "see" the next AP in the system. That usually means on a power pole or light pole or a user supplied pole of some sort! It means you will have to have POWER at each unit as well.
You can't legally run more than the maximum power allowed. You will have to check the FCC regulations and use the limits set. (2watts is well over the legal limit! 1watt is barely under and will only allow a small antenna!)
You will need an internet connection LARGE enough to handle the users! (Multiple DSL's or T lines but with enough bandwidth to allow all to connect.) You also need BUSINESS class lines so that you aren't violating the ISP's terms of use policies.
Here are some places that supply commercial grade wifi gear. You want to find "Mesh" or you will have to build a large control center to integrate all the AP's.
http://streakwave.com
http://moonblink.com
http://www.motorola.com/business/v/index.jsp?vgnextoid=91c623805ae46110VgnVCM1000008406b00aRCRD
http://www.azotel.com/?gclid=CKXomfqurJgCFSbCDAodh2NqnQ
http://www.hol4g.com/mk/index.aspx
http://www.wisp-router.com/news.php?gclid=COL90ZavrJgCFQECGgodqnfHmA
Do more research - if it were easy everyone would do it! There are tons of sites with information for you. But it is not quick, not easy, not cheap!
Read these
http://www.broadbandreports.com/forum/r20699195-New-WISP-setup-complete-6-stories-up
http://www.isp-planet.com/fixed_wireless/business/2002/part15org.html