look into the Flowmaster pipes. I called them and talked to one of the salesmen for over 30 minutes. Very impressive. He talked about the testing/research that is done and it made a believer of me. They also have some tech tips on their site
http://www.flowmastermufflers.com/cgi-bin/flowmaster
they talk about pipe diameter (our little motors only flow so much volume), the correct lenght for tail pipe pieces, etc. He says their system was almost too quiet when they tested it.
They offer a true dual exhaust, mandrel bent, and 10-15 year lifespan materials. they sell for about the going rate, $375 for cat back.
Mike