I'm sure someone with a lot more knowledge will be along soon, but heres my .02 cents. I would suggest at a minimun getting a Vec II or III. When replacing the headers with a 3" exhaust, it's possibly to throw an O2 sensor code. This is a problem that I am working on in another thread.
You wont need anything behind the Cats since you still have em. Some will suggest that after you get everything installed, taking it to a dyno to tune it all in.
I have B&B headers on mine, I love em and the sound is incredible. At idle in gas stations or traffic lights I get tons of comments
