Catalyst--

It all depends what you want to do with your Xterra.

If you just drive around town, then something like a daystar lift for cosmetic purposes, or a better stereo, might be the ticket. Lifted Xterras do look cool, and nice stereos are fun.

If you want to drive your Xterra off road, I'd go with sliders, a shovel, and a hi-lift as your first purchases (make sure you learn how to use a hilift correctly, though, as it can kill you if used wrong). And make sure to learn how to TREAD lightly http://www.treadlightly.org

Personally, I think the Xterra has a fine sounding exhaust as is. I can't see messing with that.