There have been a bunch of threads about this already (including 2 right below this post). You can do a search for "Flowmaster" and come up with a ton of information on opinions, difference between 40 and 50 series, size, model #'s, etc. Here's a few links to get you started. Hope this helps.

http://www.xterraownersclub.com/ubb/ultimatebb.php?ubb=get_topic;f=70;t=000685

http://www.xterraownersclub.com/ubb/ultimatebb.php?ubb=get_topic;f=70;t=000675

http://www.xterraownersclub.com/cgi-local/ultimatebb.cgi?ubb=get_topic;f=70;t=000626

http://www.xterraownersclub.com/cgi-local/ultimatebb.cgi?ubb=get_topic;f=70;t=000607