Apple iMac (21.5 and 27-inch, Late 2009) - Manual (Page 36)

36 Chapter 3 Boost Your Memory
Your iMac comes with at least 4 gigabytes (GB) of Double Data Rate 3 (DDR3) Dynamic
Random Access memory (DRAM) installed as two 2 GB memory modules. You can
replace or install additional 2 GB or 4 GB memory modules for up to a maximum of
16 GB. Memory modules must meet the following specifications:
 Small Outline Dual Inline Memory Module (SO-DIMM) format
 1066 MHz, PC3-8500, DDR3 compliant (also referred to as DDR3 1066)
 Unbuffered and unregistered
WARNING: Apple recommends that you have an Apple-certified technician install
memory. Consult the service and support information that came with your iMac for
information about how to contact Apple for service. If you attempt to install memory
and damage your equipment, that damage isn’t covered by the limited warranty on
your iMac.