Not sure about the torque spec, doesn't take much with the crush washers. A bit of copper antisieze on the treads is good for next time. Soak the plug base in liquid wrench for a couple hours if it doesn't come out easy.
I've heard the #6 plug on the Xterra (much like the Toyota 3VZE V6) is a ROYAL pain in the <img src="/forums/images/graemlins/butwiggle.gif" alt="" /> to get a socket on. You might need a couple universal joints and extensions.