If the check engine light isn't on then the cat is probably okay.  I know CA has some different laws regarding emissions.  On ours I had a muffler shop put on a universal replacement cat, it cost about $120.  If you're talking about the exhaust from the cat on back, I had the Banks system put on later mostly because I was losing a muffler every year and the Banks is stainless steel with a 5 year guarantee.  It's awful loud though, even for me.  Got the Banks header installed later which probably doesn't help.  The header is more hassle than it's worth.
As far as I know, Banks parts are all CA legal.