I just got all four for mine since it has 141,xxx miles on it and I am getting the PO420 H C code.
I have been searching around because I was wanting to replace all four and here you can get the upstream for $28 either in the Bosch or Denso flavor and the downstream for $51 the same flavors.
Good deal IMO.
So if you are high mileage you may want to look into the upstream even if you arent throwing a code just to natbe get back some fuel economy. The upstream is the one that regulates the fuel and spark and if they get old they can switch slow from my understanding.
Hope this helps some one looking to replace them anyway.