Well, you can check out alot of Black Sabbath's stuff- much of it is very simple in concept, but will teach you good rythm and how to layer different ideas into a song. Also, most of AC/DC's rythm work is very basic- painfully so for me actually. Basicly keep an ear out, play the stuff you're interested in learning, you'll begin to figure out what's in your reach now. Also, you'll find that challenging yourself a bit will help your playing loads. Try inventing a riff you can't quite play and practice it until it works.