Get two Buffalo WHR-HP-G54. Set one as a main Router, and the second as a Repeater.
The source Router can stay as is, the second has to be flashed with Tomato, or DD-WRT.
Buffalo WHR-HP-G54 -
http://www.newegg.com/Product/Produc...N82E1683316213
Note, if the repeater does not provide the coverage that you need, then use
kevnich2 advice as posted above.
WDS -DD-WRT,
http://www.dd-wrt.com/wiki/index.php/WDS
WDS - Tomato,
http://www.myopenrouter.com/blog/entry/10354/Configure-WDS-with-Tomato-Firmware/
WDS
The general approach that I take for Coverage issues is the following.
The best way is to lay few CAT6 cables to central locations in the house, install Access Points, or Cable/DSL Routers configured as an Access Points ( Using a Wireless Cable/DSL Router as a Switch with an Access Point -
http://www.ezlan.net/router_AP.html ), and connect them to the Main Router.
You/your client do not want/can not/hate to lay Network Cables.
Start with One affordable Router that can Do WDS (the reason for the WDS support is in case you need to add more Wireless hardware).
If you are lucky and your environment is conducive to get covered with one Wireless Router you are done.
Otherwise, using a Laptop loaded with Netstumbler, do a Wireless survey in the house,
http://www.netstumbler.com/downloads/
According to the Netstumbler's signal strength reading, identify spots that have strong signal. and spot with weak, or No signal.
Evaluate how you can cover the space and start placing WDS units.
Additional Wireless Routers in WDS Mode (Wireless Network - Configuration Modes. ) has to be placed in spots were the signal is good about Half way to the dead spots.
How many WDS units are needed? It depends on your specific environment (that is a good the reason to buying WDS units one at the time, try it, and decide on the Next step).
Otherwise.
Extending Distance -
http://www.ezlan.net/Distance.html
Wireless Router as an AP -
http://www.ezlan.net/router_AP.html
Wireless Modes -
http://www.ezlan.net/Wireless_Modes.html
Wireless Bridging -
http://www.ezlan.net/bridging.html
Hi Gain Antenna -
http://www.ezlan.net/antennae.html