I thought that "power chords" were the first and the fifth NOTE of a scale - not separated by 5 semitones, so for example 5th string 3rd fret (C), and 4th string 5th fret (G), and if you were going to go REALLY mad and make it a three note chord you could put in the octave of the first note - 3rd string 5th fret (C again).
If you are only using two notes is it still a DIAD - meaning two notes, rather than TRIAD meaning three notes?
Help.