It's usually best to run a distortion box into a clean amp- basicly if you run a distortion box into a distorted amp the two distortions add together. Sometimes this is what you want (the Smashing Pumpkins have used some insane number of Big Muff Pi's (like 30?) for one song), but usually that much distortion is very undesirable. If your amp's distortion is footswitchable, you can certainly use the two together:

• Just the distortion box for heavy distortion
  


• Just the amp distortion for overdrive
  


• Both for fuzz
  

Regardless, a distortion box is a good idea. I recomend a BOSS DS-1, but shop around and see what you like. Another peice of advice: UNDER NO CIRCUMSTANCES BUY A DOD PEDAL- the switch won't work right in 6 months because they use a cheap mechanism that wears out very quickly.