http://www.flowmastermufflers.com/cgi-bin/flowmaster/super40.html

what is up with this, which 40 series is the best to put on the xterra?

You can pick them up for $80 on Ebay. If you take it to any muffler shop it doesn't matter to much if what kind of In / Out option you select Center In / Center Out etc. as long as you go with the 2.5" diameter In option, which I guess is the standard diameter of the pipe already there (Makes it easier for the installer). $40 / $50 installation at most Muffler shops.