iMac Early 2009 restarting after RAM upgrade
Remove one of the 4 GB strips of RAM so that you are only using half of the total 8 GB that you say you purchased. Does the system run stable? If not, replace that strip of RAM with the one you removed. If the system is still not stable, remove that strip from the slot and insert it into the other slot. If this still does not solve the problem, you may want to try a different brand of RAM.