Several times there were questions whether late 2009 iMac i5 and i7 models really require 1066 MHz memory modules (as shipped with and as specified by Apple and other aftermarket vendors) or whether it can be upgraded with faster 1333 MHz modules. I ordered generic 2x4 GB 1333 MHz kit from ebay (CL9 Hynix branded chips) and I can confirm that they work both in 1333 MHz mode (when installed standalone) and in 1066 MHz mode (when mixed with original 2x2GB 1066 modules). See the screenshots (first one there were just 2x4GB 1333 MHz modules and on second there were 2x4GB 1333 MHz + 2x2GB 10666 MHz). I will do some benchmarks to see if there are any benefits of having faster modules, but I expect just small benefit. Since there is no price difference between 1066 and 1333 modules I suggest to buy faster modules even for 2009 iMac's (i5 or i7 only, Core2Duo can be still limited by chipset).