hi power wifi doesn't exist for home use. If you think about it, your computer would also need to have a high power transmitter to get the signal back to the base station. The best way to extend range is to use something called a 'repeater.' Apple airport expresses are easy to set up in this mode. Other routers can also be set up as repeaters. They act as nodes that can communicate with your main router and effectively extend the range of your wifi.

Your best bet is to get one (or more) used apple expresses from ebay. It is pretty straight forward to set this up. I would rate it much much easier than changing the front CV boot or wheel bearings =). About as hard as a change of plugs.