Stangirl2000:
I had to laugh at what you said about not wanting your Mustang to go much faster, because I was worried mine wouldn't go fast enough...until two hours after I got it. I was clocked at 101 mph by Florida's finest, so I know the little V-6 will provide plenty of bang for the buck.
As for the sound improvement, everyone I've talked to has suggested the Flowmaster exhasut. However, my dealer said my warranty would be void if I used anything outside of Ford (installed) parts. You might want to check this out to be safe, though I think it would be worth the money.
A source that has been suggested is:
http://www.drl2.com/Mustang6/
Pastor Robert