A lot of Nirvana songs are pretty power chord heavy. Not punk of course, but they should help you learn to get into that technique rather easily.

Some good songs to work on your chord changes:

Hotel California
Under the Bridge (and some other RHCP songs)
Lenny by SRV - the main riff, don't even try to learn the whole thing at this point.
Babe I'm gonna leave you (strum the chords if you can't finger pick)

I'll look into these. Anyone else?

yesterday, dust in the wind

Not punk

3 chord guitar.com or .net has really helpful videos and he picks a nice variety of songs.
P.S. dont be picky about what you learn at first, you need as much practice as possible and you should try to play songs from every genre to develop your own style.

nothing else matters- metallica

yea you should maybe think of playing some other stuff even if you love punk so much, especially if your playing chords, open or power chords. other sort of styles will show you different techniques but if you stick to punk thats all you'll know.

ive been playing maybe over a year now, some good songs i play for chord changing etc. are:

sweet home alabama - lynard skynard
hey joe - jimi hendrix
whatever - oasis
put your lights on - carlos santana
zephr song - RHCP

i know there probably not your style but i hope it helps

When i Come around - green day


easy song, completely power chords which will help your chords a lot, also a super-easy solo to play, also some palm muting on power chords during the main riff if you need practice on that,