Decat isn't worth the issues (read: big ass fine) you'll get from the cops if they catch you out. And being on ur ps ull get pulled over... A lot.
Best way, go to ur local exhaust shop and ask for a mandrel bent 2 1/2 inch system out of mild steel with a resonator and an oval (not canon style) rear muffler.
1. It won't be legal db wise (legal noise is practically silent)
2. It won't net you huge power gains
3. Please learn to use the search function as this has been answered many times before
4. Don't come back in A months time and complain that it's Droney, audibly loud natro skylines will be Droney on the highway, don't like it keep it quiet it's as simple as that.