Best stomp box I've used is the Line 6 DM4, it has all the big distortion types including the Metal Zone, Ibanez Screamer, Tube Screamer, Overdrive, Fuzz, etc etc. Worth checking it out at the local guitar shop. You should be able to get most guitar sounds out of this unit. You won't actually nail Metallica's tone unless you go through tube driven hardware though... but you might get close.