In regards to the Nissan Xterra's exhaust, the worst part on it would have to the Y-pipe. Who ever is plaining on doing an exhaust install: It'd be smart if you cut out the y-pipe by either having a dual in muffler, and single out. Or simply having dual exhaust after the cats.

Out of all the efficiency loses that the exhaust produces on the engine, the Y-pipe (I believe) is the worst.

If you want a mixture of power increase and simplicity, then why not have a dual in muffler, and a single 3" exhaust. I'm going for dual glasspack