I heartily agree with both the antenna and DD-WRT suggestions. DD-WRT is a terrific router operating system (as is Tomato) and I use it on all my routers. If you are uncomfortable changing the router operating system, you can buy routers already converted to DD-WRT on eBay. My favorites are the Buffalo routers.

eBay is also the place to buy router antennas. You have to be careful about the antenna connectors, as there is no standardization and the terms are incomprehensible ("reversve polarity TNC", for example) but you can tell the antenna vendor what you want to connect it to and they can usually provide the necessary cable. The antenna is the way to go if signals are weak because, as pointed out above, it amplifies both received and transmitted signals. A word of caution: directional antennas work by ignoring all but one direction so don't assume you will get signals off the back and sides of the antenna as well as the front.

Using 2 Buffalo routers running DD-WRT and 14 db Yagi antennas, a client runs a nightly off-site backup to a location over a mile away (line of site).