Yeah, those power chords are a bear! The most important thing is that you understand how and why they work they way they do, as regards the logic of the guitar neck. Don't drive yourself crazy trying to get perfect tone at this point. Focus more on position, location on the neck, and why those chords are named the way they are, and how they relate to other chords ("The Magic L", for instance).

You'll get a lot of mileage just out of understanding those things - even if your tone is not perfect.

-Lisa