Expanding wifi range can be accomplished in a number of ways. The easiest is to use a repeater. These devices connect to an existing wireless network and broadcast a new one that is connected to your main network. The advantage of these is simplicity in setup. The downside is that they take a performance hit and usually have you broadcast an additional network. name (SSID) so to not conflict with your main. But they get the job done.
Look here for Netgear's range of models. http://www.netgear.com/home/products/networking/wifi-range-extenders/ Note there are dual band N models that if you have a dual band N based router you can share the load of transmit and recieve between both bands thus really improving performance. That said...