Buy it and buy it fast. That's the same iMac I have that runs amazing and I bought it for $1800 in August of 2011. You'll be getting it at under half price for a year old machine really. They run amazing and well worth saving the money.
As far as RAM goes, it can hold up to 32GB. As people have mentioned though it CAN be unstable so unless you really need that much RAM I would stick with 16GB.
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That is a lie. This RAM will work perfectly fine. You don't ned "Apple Qualified RAM" you just need RAM with the same specs. if you look in the feedback people point out that the RAM posted works fine in 2011 macs. Do your research before giving advice.