You could get a pretty cheap plan for a high gain antenna off of "instructables.com". God I love that site! Total McGyver stuff.
I would look into the plans for a WOK-FI homemade antenna from a wok or an old satellite dish, or a Cantenna both of which should work fine that distance.
Another buzzword to search is "wardriving" which is when you search for an open network in your car, often with a pringle can antenna. They are all about getting the best signal from your neighbors, so there will be a lot of info about the best routers, tricks etc. to getting the most bars. The first and easiest way to increase your range may be to put your router as high as possible, in the attic if possible.
I just bought a Verizon 7501 router on the Verizon website for $10, which has a reputation for a crazy good signal for the price, and suddenly noticed I could pick up 3 times as many networks in my neighborhood. It works for anyone not just verizon.
Looks like if you are willing to do some McGyver work, you should be able to share that connection and drop the landline!
BOB