If you have a smartphone you can get apps that will check the different wifi channels in use. Set your router to the least used channel.
For Android, WiFi Analyzer is a good app. If you start it up you'll see what I mean by channels in use.
For 2.4Ghz, channels to use are 1,6, or 11.
Not many people use the 5Ghz channels yet, so good to use that.
Some devices cannot use the 5ghz channels yet however, and 5Ghz does not go thru walls as well.
If you are getting an AC300 adapter, you'll be able to connect to just about anything (backwards compatible to older networks).