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or what is the best processing power CPU I can use on my Late 2014 27" iMac? my current CPU is the i5 (I5-4690). I’ve checked the i7 version is using i7 (I7-4790K). Does it mean the best i7 CPU I could get into the iMac is the 4790k? Thanks!

Yes, the best your system can support is the I7-4790K.