MacSales is selling a 4x8GB kit for $160.00 and Crucial for $198.99. I would recommend 32GB (4x8GB) which will set you back less than $200 and in some cases less than $150. There is no point in running faster SO-DIMMs, besides, DDR3-1866 DIMMs should be fairly cheap. There are threads on these forums that can give you all the information that you need. You also have a proprietary PCIe SSD connector in there that you can put in an Apple PCIe Flash Storage SSD and in some cases, a third party SSD using an adapter. If you swap out the mechanical HDD for a 2.5“ SSD you should not have to worry about the fans going crazy. The 6700K is faster overall with 4c/8t, but not necessarily a huge difference maker in day to day tasks the way the SSD will.

The speed difference depends on the applications you run. The 8th and 9th Gens are not electrically compatible, despite using the same LGA-1151 socket. While a Core i7-700K is technically possibly, I don’t know anyone who has done it.Apple’s EFI is most likely going to prevent that. You cannot put a 7th, 8th or 9th Gen CPU in this computer.

Look on eBay and you find a good deal, or even an off-lease PC that you can swap CPUs out. The cost for those in any sort of new condition is between $350-$400, so it’s an expensive upgrade for a 4 year old machine. The i7-6700K is the fastest that you can get for the Late 2015.

How much memory can late 2013 imac take