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iMac 5K (late 2014): Memory
The MPG iMac 5K can accept upt to 32GB of memory. 64GB is not possible due to hardware limitations.
OWC has 8GB, 16GB and 32GB memory kits for the iMac 5K and the existing memory can be sent in for a OWC memory rebate. This nets out the cost of 32GB to about $358 (versus $600 at Apple, which pays for the faster CPU option!).