1998 Honda Accord Question: "A & B Pipe" with Catalytic Converter?

Hi,
Recently my "check engine" light lit up and was diagnosed as needing a new catalytic converter. I was quoted $700+tax. Then they came back to tell me that since the "A & B Pipes" had been previously welded together, I needed to have those replaced as well. This would now cost me $1,700+tax. H have no idea what the A & B Pipes are or how they got cut apart. The mechanic said some muffler shop probably cut them apart. I have owned the car for 9 years now and never hear never taken it to a muffler shop. I HAVE had the ERG (?) valve cleaned the last 2 times the check engine light went off. That seemed to fix the problem then. Since I live in Asheville, NC, we are required to have an emmissions inspection (next due in September) in order to keep our registration.
I guess my question is this: what are the A & B Pipes and do they along with the catalytic converter HAVE to be replaced? Does the ERG sensor have anything to do with this? How much is a reasonable price to pay for all of this? Help!