I noticed in one of the threads that some people said getting a new muffler is a cheap way to increase the Xterra's power. So how much can the power increase by installing a better muffler? Would it even be that noticeable? Also, how would it effect gas mileage?

My bad, I should have checked in the Performance section, it looks like they have some muffler discussions already

many people say to stay awa from calmini especially if you live in the northeast because they have been known to crack... i live in southern conencticut and it gets to -10 here nad i havent had any problems with mine. i have had my calmini exhaust for almost a year now and the power difference is noticeable. i also get about 10-20 more miles to the tank...but then again i have a k&N cold air intake system as well... at any rate for the price the calmini is unbeatable.