It depends on what u want. If u want loud or a little more mellow. Most of the people on here go with duals and flowmaster exhaust. Thats prolly one of the loudest, meanest sounding. IMO i like the lower rumble, not so loud in the cab sound. SO i am gonna go with either the borla or magnaflow soon.
The only part of the exhaust system that needs to be changed (besides the exhaust manifolds and y-pipe) is the muffler. You can just replace the muffler, you don't need a catback. Here's what I have gathered about mufflers, the rest is up to your preference:
Borla & Magnaflow-high quality straight-through mufflers that have a more mellow, but still powerful sound
Flowmaster 40-about the loudest performance muffler you can get, you might run into drone at some rpms
Flowmaster 50&70-quieter than the 40
Glasspacks are generally pretty loud, probably about on par with a flowmaster 40 series. The muffler I would suggest would be the magnaflow or borla. If they are too expensive and you don't mind having a loud muffler, go with a FM40 or a glasspack.