Neither of them would really be bottlenecks. Your i7 2600K is still a great processor, especially if overclocked. Also, 8GB of RAM won't stop you from attaining 60FPS in most if not all games, maybe except for Batman Arkham Knight. That game on PC is a unoptimized buggy piece of shit that they recommend having 12GB of RAM for just so it won't stutter as much.
What case do you have right now? You listed the Corsair 750D a short while ago in your parts list, but I think you have a different case at the moment? The Define R5 is a very nice case, just about every professional review you can find online is glowing with praise. Airflow is pretty good, it'll take 140mm fans in just about every mounting spot and doesn't suffer for high temperatures or poor ventilation despite being fitted for noise-suppressing foam all over.
Right now I have an Antec 1200v3. It's a nice case, just loud.
I'm going to get a christmas bonus at work this year, so that's why I was thinking of future proofing now while I have the money, but if it's not worth it, maybe I won't. I built this guy in 2011. Just Cause 3 is making me think a CPU upgrade would be worthwhile...