I have a 2003 xterra and am looking for more hp and better gas mileage. I've decided to upgrade my stock muffler and pipe. What have you guys used and what do you recommend? I'd like to keep the cabin noise down if possible... Thanks!

I've always been partial to magnaflow mufflers.

Yea, I like the Magnaflow too.
I bought one that's 21 inches long X 6 inches round with 2 3/4 inch in and out tube. Sound GOOD !

Your not going to notice any reals gains off just a performance muffler. You'd have to do a series of other items as discussed previously to gain any real HP.