How can you increase the wireless coverage area?

You can buy more powerful router. Also some routers allow to have bigger antennas but not all of them. You will have to upgrade your wireless cards because it should have also strong enough signal.

But there exists better technology and it's called bridging. To start using that you will need two routers which support bridging, and they have to be from the same brand. After you have it just read manual how to set up bridging configuration.