You would know if its the upper manifiold O2 if your MPG has been continuing to drop since the light came on.
The rear O2, although IMO usless, acts as a warning system and will still set a check engine light (Code P240)... and cause you to fail inspection in most citys/states
Find out which one it is an replace it... if you have a 00-03 then you can buy a o2 simulator for the rear one... its a little cheaper and if you ever get a downpipe then you have already gotten rid of the code that it will throw
Also... your are correct about the AC delco thing... go with the OEM... the bosch's are crap for most GM applications
