RAM & Memory Upgrades
OWC - 16GB OWC Memory Upgrade Kit - 4 x 4GB PC8500 DDR3 1066MHz SO-DIMMs for all Apple iMac 21.5" and 27" (October/2009) models
OWC Memory Upgrade Kit: 16.0GB (4x 4GB) PC8500 DDR3 Non ECC 1066 MHz 204 pin SO-DIMM High Quality 16.0 GB (4x 4GB) OWC Brand PC8500 DDR3 Non ECC 1066 MHz 204 pin SO-DIMM Memory upgrade kit Compatible with iMac 21.5" and 2" (October/2009) Models...
https://www.bizrate.co.uk/ram_memoryupgrades/owc-16gb-owc-memory-upgrade-oid6878399969.html