Best Deal from Amazon.com : https://www.amazon.com/gp/product/B0091L3QIC?tag=chriskressera-20
4GB Upgrade for a Apple iMac 28GHz IntelQuad Core i5 27inch DDR3 Mid 2010 System
Product Description :
204-pin SODIMM
DDR3-1333
DDR3 PC3-10600 Unbuffered NON-ECC DDR3-1333 135V 512Meg x 64
The only brand of memory upgrade available directly from a DRAM manufacturer tested to meet the most stringent industry specifications
Install it yourself in about 15 minutes The same top-quality memory that the worlds largest computer manufacturers install in the systems they sell