The best way to improve the range is to add a wireless access point (cheap N router configured as an AP) that is connected to your main router by an Ethernet cable. If that is not possible, then you would need to use a pair of MOCA adapters if you have coax cable in the walls or if your bandwidth needs aren't too great powerline adapters on your electrical sockets to connect the router to the AP. Repeaters are usually not a very good solution.
One major limitation on your wireless coverage is other local wireless networks, which you can analyze with a freeware called inSSIDer
installed on one of your wireless laptops. It will show all local networks by signal strength and channel to help you optimize your wireless.
802.11ac is starting to come out, but it is still expensive and not really fully baked. That said, for about $380 you can get a pair of the ASUS routers and configure one as a media bridge to extend your signal quite far as the recent Tom's Article