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:
Additional notes i5 and i7 Quad Core iMac computers come with both top memory slots populated. These computers will not start up if only a single DIMM is installed in any bottom slot; these computers should operate normally with a single DIMM installed in any top slot. Core Duo iMac computers should operate normally with a single DIMM installed in any slot, top or bottom. ("Top" and "bottom" slots refer to the orientation of the slots in the pictures below. "Top" refers to the slots closest to the display; "bottom" refers to the slots closest to the stand)
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).