Stone building with reinforced concrete floor, or wood?
If you get a new router: try to run a wire as far as you can for the old router to function as a range extender.
If you have a smartphone, there are Wifi analyser apps, so you can check which channels are the least crowded.
The ASUS RT-N66U is considered one of the best you can get. It's "802.11n", but an "802.11ac" router has less range.
To boost your reception and transmission of the wifi signal now, have some fun with the Windsurfer. Google it. :)
If you have more than one antenna on the router, put them at 90 degree angles to one another.