It'll probably be the rear O2 sensor as that is used to measure cat efficiency. But if you haven't changed your front O2 sensor till now then that might be a good thing to change as well. You can hook the front one up to an oscilloscope and have a look at its response time, I just replaced mine and it's crossing the 0.45v line about 5 times more frequently. From memory the new O2 sensors have an average life of 130k km, or 80k miles.
As for OBDII and cat efficiency:
"The OBD II system compares O2 sensor readings upstream and downstream of the converter. If the downstream readings start to match those of the upstream O2 sensor, it indicates a drop off in operating efficiency and sets a catalyst code P0420."