By federal law you need a cat. Though many still don't run them.

Your car only has one cat, and removing it will gain you nothing. So its best to keep it for sake of never having the check engine light come on.

Straight piping will be very loud and after a little bit it will grow old and you'll want to add a set of mufflers to tone the sound down some. I'd just pick a pair of mufflers and weld then in. Keep the resonator so you don't get drone.