Generally speaking to improve on hp or mpg you
need a nice free flow muffler. Some of the best performance
mufflers are straight thru design. Ie Magnaflow and some
others. How they pack the muffler overall length of the muffler
generally determine how loud it will be.
Magnaflows are packed with stainless steel. Last longer and does
better at quieting things up then something such as the old
fiberglass packed mufflers, ie turbo mufflers etc.
I suggest something that should be the best of both worlds is
this one:

http://www.magnaflow.com/02product/s...ne=main&id=459

Covers all the bases and well built and cost under $100.
If quiet is a MUST, then we offer and most shops around all offer
the old school turbo style muffler with an S pattern tube inside for
flow and glass packed, avg cost about $50. The S pattern inside the muffler
flows a little better then stock, but still packed with fiberglass it will be
pretty quiet. Problem is over a few yrs time, it tends to get louder with age.
The Magnaflow wont do that due to its stainless guts.