You can use the NS-2 on its own (I have). Next time you're playing your amp that loud, try rolling off your guitar's volume. Does that get rid of the noise you're talking about? If so, an NS-2 in front of the amp will do the trick. I just had the threshold all the way up and the decay all the way down on mine and it worked great. I got an ISP Decimator recently and think I like it more, but for what it is and how much it costs, the NS-2 is great.
EDIT: In my experience, I have yet to find a distortion pedal that does what a good high gain amp can do on its own. However, it's worth looking into a Tubescreamer/Tubescreamer-style overdrive (BBE Green Screamer and Digitech Bad Monkey are some more affordable ones that do the same thing) to put in front of your amp. It hits the front of your amp harder and tightens your gain up and makes it more aggressive, while letting you back off on the gain on the amp. Not everyone prefers this, but a lot of people do, and I can't live without a boost now.
If you get the NS-2, you would go Guitar --> OD --> NS-2 --> Amp. The most common overdrive setting for this is to turn the volume all the way up, the gain all the way down, and tone to taste (I keep it around noon).
"it blowed the emg's in the dirt" - De Vries