OK you should check out this site
http://www.wardrive.net/ for info on making all types of wifi antenna's.
here is a antenna i think would work nicely for what you are trying to acomplish http://www.seattlewireless.net/index.cgi/CardboardHorn
now to make this a even cheaper antenna you can just place the rubber duck antenna that is on your router right in the box at the end of the horn antenna...here is a nice crappy paint illustration
as for what hardware to use you could use any router and buy a wifi repeater but remember they do not have a vary high power output so you might want to just spend the money to get two
linksys wrt54g's.
For testing you could just make one horn antenna and then point your router/antenna down the street and walk down the street with a laptop/pda to test how much range you are getting from it. now remember this will work much better when you have good antenna's at both end of your link.
