I’ve got an iMac Mid 2011 with 4GB RAM preinstalled (2 x 2GB) and want to upgrade to 12GB. Is it ok to add a single 8GB RAM or is it better to add 2 x 4GB?
Single DIMMs might not be supported, depending on the model of iMac you have. According to this Apple KB:
As a side note, 8GB DIMMS are not officially supported - only 2 or 4 GB SO-DIMMs. However, according to this OWC page, 8GB DIMMs do work (4 x 8GB).