2002 Mitsubishi Eclipse P0421

I have this Mitsubishi eclipse rs which I purchased about a month ago and everything is working except I have a code P0421.

I had it checked by the dealer and they said that I would need to replace my catalytic converter and when I asked which one they could not tell me, one of the other mechanics there said I would probably have to change the one up front and the other one that looks like part of the exhaust system.

Is that correct? Paying almost $2400 for the repair is awful high!

Thanks

BTW: The company I purchased the car from said that they had changed the front O2 sensor already.