I highly sugest learning some Lamb of God, As I Lay Dying, or Slayer riffs to work on metal rythm. If your having trouble with fast palm mutting and rythm in general, this will be equally tough and rewarding. I started off with metal rythm by jamming along to the newer As I Lay Dying cd since most of it is drop c power chord chugging mixed with a few notes so I learned a lot by ear. It was hard, but I was determined to be able to play like that. Now, my picking hand is a beast.

Also, sitting there chugging away with only your picking hand will get boring and make you lose intrest quickly; not to mention, you need to keep your hand syncronised just like with speedy licks.