Only the upstream affect engine efficiency.
If the downstream is bad or senses an inefficient cat, it will set a CEL/code saying so.
Upstreams get lazy over the years and the computer is designed to adapt to that, but fuel mileage will go down as the fuel trims go up as a result.
In short, replace only the upstreams for gas mileage improvement.
Reset the computer by disconnecting the battery and turning on/off the headlights.
Old HEGO sensors (should be replaced every 60,000 miles) from http://fordfuelinjection.com/?p=31
Bosch O2's are good too. Price them locally and at RockAuto.com for reference prices.