As I mentioned above in the section about P0300, my 2000 SE had a coil pack go bad and I continued to drive the car home. Now the P0420 code appeared. Due to the prior issue, I knew the CAT was now bad.
It is costly to replace the CAT, but it turned out to be necessary. Now the car is fine, but my wallet is much lighter.
Remember to STOP if the CEL FLASHES at you!