Forget about using 5Ghz it's only good for small apartment.

To get stable wireless internet while making it simple and cost effective, just add a wireless AP. Some cheap router such as tenda F3 and Fh456 have the wireless AP feature built-in and It will allow you to play with the QOS limiting bandwidth by MAC. I have a 1 month used tenda Fh456 to let go cheap if you are interested.

The drawback with wireless AP is that you won't be able to utilize the 50mbps at 1 client when sharing. Anyway, investing in so called high gain router will not ensure you to utilize the 50 mbps as well when your room is at another end of second floor.