Is Memory Upgrade DDR3 PC3-10666 the same as DDR3 PC3-10600 for 27" mid 2010 iMac?

Save yourself a lot of trouble and by ram from either OWC http://eshop.macsales.com/ or Crucial http://www.crucial.com/ Other brands aren't manufactured to exact specifications and will likely cause problems. Macs require exact spec ram.

What is the brand of the ram you're looking at?

Mac's are very picky and many off brand PC modules will not work.

see > iMac: How to remove or install memory - Apple Support

To get the best results, you should always buy from a supplier like OWC or Crucialthat actually test their RAM in the different Mac models.

PNY is one of the bargain basement brands you want to avoid.

I always say that the sweetness of saving a few bucks is quickly replaced by the bitterness of kernel panics and possible data loss.