2003 Impala - Catalytic Converter

my car had been running terribly and after much checking under the hood and around the car,  my mechanic has come to the conclusion that my catalytic converter needs to be replaced.  because they do not normally do exhaust work, he estimates it will cost between $700 and $800 for him to do it.  meineke quoted me $440.  so i have a couple of questions.  how hard is this repair to do myself?  is the cat bolt in or weld on?  where i can i find a quality brand new cat for not a lot of money?  anyone in the Northern NJ area who wants to do a fella a favor and do this for me?  maybe we can work some kind of trade off out.  incidentally, it have the 3.4L V6 with NE Emissions.  any help at all is appreciated.