Im looking into the Boss CS-3 Compressor/Sustainer and I was wondering how it would actually help my overall sound what exactly does it do. I'd like to think I have an awesome tone for what I play but how would this help me.
i know that it will crunch up your tone a little bit and give it more of a metal-ish tone.

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It boosts quiet signals and quiets strong signals, creating and overall stability of volume. Kind of (like the other guy said) crunches your low and high tones together. Also increases sustain, but that's a duh.
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i heard if you have a comp. sus. you need a noise sup. also
is this true, or why do so many people have both?
the CS-3's can have a bit of background hiss when on but i'm not sure how loud it is and if it would affect your playing.

try goin for the Boss CS-2 on eBay
I've got a CS-3, and the noise isn't bad. I use mine mostly clean. It will not give you a more metal- ish tone unless you have the gain very high in the first place. It's great behind a wah or in front of an OD or boost. If you want to use it with a lot of gain and/or pedals you might need a noise gate, it does get a bit loud boosted.