Just the muffler? As in just a rear section? If you're going to do that, I say you save your money or get some other mod. It's basically changing it just for sound/looks if you're getting a rear section.
And if you change to a full system, unless you get a muffler that is big (with lots of muffling insulation), you're going to get a louder sound than stock. Most exhausts these days make power because of the straight through flows and the fact that they don't contain a lot of muffling insulation.