5.7 hemi idle and backfire

while keeping on the subject of o2 sensors , from what i have heard that all mopars like oem sensors and that we should not use aftermarket sensors , no only have i heard this but a friend experienced it first hand and with a hemi , was running ok , then like crap , changed o2 sensors with aftermarket, ran ok for a short while , then back to crap , scratched head for while , changed computer , no change , finally to it to shop , found out aftermarket o2 sensor wasnt sending the right amount of voltage to computer , and he installed new oem o2 sensors and the thing runs spot on . so any feedback to this would be great. i am getting ready to buy o2 sensors does anyone know if you buy the down stream or up stream sensor if you are installing in the header and does the header side matter or do you install one in both ? thanks ed aka dartman 440