I put in a magnaflow performance muffler and ran a straight pipe in place of the stock resonator. It sounds way better. For a couple weeks I had only replaced the muffler and the resonator gave some weird 'loopy' type sounds combined with a different muffler.
In my opinion the $800 kits to replace the whole system are kind of a waste of money.
The biggest restrictions in our exhaust system is the exhaust manifolds, catalytic converters, huge mufflers, and resonators. You can't get rid of the catalytic converters in NY so replacing the muffler with a straight-through unit and getting rid of the resonator probably gave me similar results in HP/Torque that an entire system would have... for 1/8th of the cost. Maybe not QUITE as good but it was still noticeable enough to me and I highly doubt it is that much different.
Headers would be nice but I've only seen one company that makes them for the 3.7L and they are $$$. At the end of they day it is still a 3.7L
Different muffler, different resonator, or running just straight pipe is our best and most economical options.