get a magnaflow muffler from amazon.com for ~$90 and have a shop weld it on for ~$30. it'll change the sound of your car, and you don't have to worry about possible lossese in power from too large of piping.

 

is the magnaflow really loud, because i want one that is a little better at idle, but is really noticable at open throttle.

 

I've had a full Magnaflow catback system for about a month. It's got a subtle rumble at idle, but once you punch it, it has a nice, deep sound. I would recommend a whole catback vs. only the muffler. You won't regret it.