IMac mid 2011 RAM upgrade compatability

The iMac Mid-2010 supports DDR3 10666 (from the Apple support site)

The first link is to the correct RAM for that Front Side Bus (FSB) speed The second link is to a slightly quicker set of RAM, but runs on a lower FSB.

The differences between the two are characterised by the two notes above, the FSB speed and the clock timings.

In the first, the FSB speed is higher, but the timings for the RAM are slower. In the second, the slower FSB allows the RAM to work at slightly faster timings.

If you want to know about timings, a search for RAS CAS delay is a good starting point.

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).