RAM upgrade – iMac Mid 2011
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).