When I had my ZJ w/I6 4.0 I put on a Banks header and had a custom 2.5" exhaust built by the local Meineke shop. It sounded awesome (P.O'd my sister - gotta love it). Also had a Fastman TB and K&N CAI on the intake, so the whole package was the upgrade not just the pipes. I only spent about $350 on the exhaust itself, and i didn't have to get dirty.
If you want to upgrade your 3.7, a cat-back is PART of the picture but not the sum of it. And you don't want huge pipes either - you need a little back-pressure