I have a Mid 2010 iMac ( 21.5 Inch ) - How do I upgrade it's RAM to 16GB

iMac: How to remove or install memory

While that memory appears to be correct it is one of the brands that I consider bargain basement. The specs may or may not be exactly what they state. You be better served getting ram from a vendor that guarantees it will will work in your Mac and has a lifetime warranty such as http://www.crucial.com

just upgraded my imac mid 2011 from 4gb to 16gb using crucial and it works great

Another excellent RAM vendor (my personal preference):

http://eshop.macsales.com/shop/apple/memory/iMac

I too have a mid 2010 iMac, 27 inch and am considering upgrading to 16GB RAM. I've read a couple long threads that babowa has linked to a thread I started a few weeks ago, and they've all seemed to have problems with 16GB with the same iMacs. One user posted that 4x4G from Crucial worked fine for him. It has been a while now since those thread have been updated and I wanted to know: Would 4x4GB from Crucial work with my 2010 27" iMac? Has Apple since fixed the problem with a firmware update?

I have 4x4gb Crucial in my late 2009 27" iMac and have no problems.

Okay, I think I might just order 16G from Crucial then. If I experience any problems with the RAM or if it doesn't work properly, do they usually charge for any return fees?

Crucial customer service is excellent, there should be no fees, but you should check on their policies yourself. If you're in the US another great vendor is OWC http://eshop.macsales.comI've bought from them several times and have never been unhappy, again guarenteed product, and excellent customer service and support.

Great, thank you!