Big Muff all the way. My cousin is in a hardcore band and he uses one along with his 5-string bass. It works great on guitar too. And if you can, look into some EQ and echo pedals as well, you can get really cool sounds out of them for that kind of music.
I am in a death metal band, so I know what kind of sound you a probobly shooting for. To be honest, sometimes bass sounds best with no distortion. A clean bass tone will blend and thicken a sound very well. If you must have distortion on your bass, I can say that most pedals will give you a similar sound. While they will create all kinds of rich tones on a guitar, most will drown out into an AM radio-like sound on a bass. Go for a cheaper guitar distortion pedal to use on your bass.
E-mail me if you'd like to talk about extreme music.
Great, thanks for the advice. I really like the way a clean bass sounds too, but he's playing bass, not me. I think he'd primarily be using it for sludgier parts a la dragbody, etc...
We have an old Boss Distortion, Heavy Metal, and Metal Zone lying around, so I think we'll play around w/ those before he goes out and spends bucks.