Originally Posted by vipor

you can replace the Cat-Back Exhaust (CBE) if you want, the Nismo is quieter than most.

before you do that I would recommend replacing the Catalytic Converters with High Flow Cats (HFC.)

normally I would recommend Test Pipes instead of the HFC's or maybe even Long Tube Headers (LTH,) but based off of your apparent experience level you should opt for the legal method.

Berk, Fast Intentions (FI) and Stillen all make very popular options. look under the Intake and Exhaust category.