Look up some tabs for these songs - they all use power chords:

"Freak" by Silverchair (probably one of the simplest metal tunes ever written!)

"Anthem for the Year 2000" by Silverchair (will help you get the hang of alternating between E-string and A-string power chords)

"Peace Sells" by Megadeth (great riff, and you can practice playing in Eb)

"Whiplash" by Metallica (uses a mixture of thrashed single notes and E-string power chords, and it's great for building up your speed. I always warm up with this song before I play any heavy metal)

"Ashes of the Wake" by Lamb of God (again, uses both single notes and power chords, and the use of the Middle Eastern scale makes this an extremely interesting tune to play)

"Wicker Man" by Iron Maiden (so simple, yet so much fun!)