If you feel like taking a plunge here, you can install an alternative firmware for your routers from DD-WRT
http://www.dd-wrt.com/site/index (this includes flashing your routers, this can brick a router if not done right, and will make a router useless if bricked)
This will allow you to create a wireless repeater, a wireless bridge, and a few other things as well, eliminating the need for an Ethernet cable. The firmware only needs to be installed on the repeater, and not the main router that connects you to the internet.
With a solution like this, you would not need to run cable throughout your house.
Directions on how to create a wireless bridge from an old router
http://www.dd-wrt.com/wiki/index.php/Repeater_Bridge. Make sure your router is compatible before trying.
If you do not feel like using the DD-WRT, using Ethernet cable between routers and setting the one that does NOT connect to the internet as a access point will do the trick, as the link from a previous poster explains in detail.