This is my experience here:

Went to a local muffler shop and told them what I wanted. They had the muffler and did a custom piping going from 2.25 inches to 2.5 inches in diameter. I would not suggest going to anything more than 2.5 inches since it would drop the low end torque even more. We all know the X could use more torque.

I'm using a Flowmaster 40 Original series and I love it. It's nice and deep when I drive and a nice mellow sound when at idle. You will never achieve the nice rumble of a big V8 no matter what exhaust you chose.

I have to note that if you are looking for sound some people have said that Glasspack was a lot better. I'm happy with what I have.