I wanted to try 4k gaming but my old gtx 560 couldn't handle it... so I decided to build this rig. And it works like a charm =) Gtx 1080 was an obvious choice but I couldn't get an interesting custom card because the worthy ones are too long for this case... I overclocked the 6600k to 4.5ghz but I can't really get more without changing my air cooler for a watercooling (70C during stress test and 60C during intense gaming).
For next year upgrades i'm considering watercooling for both CPU and GPU so that i can also OC the GPU (the witcher 3 @ 4k is not always smooth with hairworks ON). I'm not really considering a RAM upgrade since the difference between 2400mhz & faster RAM is not that important in game.