I disagree. Upgrade. Both Evil Within and Lords of the Fallen showed a fat delta of 10FPS+ between Haswell and SNB. Don't be surprised if you see that gap next year with new AAA games. Plus Z68 and older platform wise is pretty meh compared to Z97.
Lords of the Fallen:
780Ti ~ GTX970 gets 72 fps min / 84 fps avg on 5960 @ 4.6Ghz
A 4.2Ghz 2500K will be very close to a stock i5 4670K in performance and faster than a stock 2600K. Basically, his CPU would completely max out the 970.
At 2560x1600, you are GPU limited with a 970.
Evil Within - very poorly optimized port with meh graphics
"Is it possible to run The Evil Within on PC at a locked 1080p60?
Not even an overclocked i7 and the fastest GPU on the planet can manage it." ~ Digital Foundry
Even a 980 with i7 3770 @ 4.3Ghz drops to 38-42 fps at 1080P. < This particular game is a slow title that provides no benefit whatsoever when running it at 60 fps vs. 40 fps. It's pointless to spend $ on a new CPU + Mobo for this title.
Z68 vs. Z97 - there is nothing really on Z97 that's
revolutionary other than PCIe 3.0 which barely matters against x16 2.0 and M.2 which is dead or alive at the moment given the lack of M.2 SSD offerings on the desktop. Unless he is running a lot of SATA 3 HDDs in Raid, he can wait for Z107.
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The 3 games really worth upgrading for from i5 2500K are Crysis 3, BF4 Multi-player and Arma 3 multiplayer. With a powerful enough GPU setup, AC Unity would benefit but a single 970 will be entirely GPU bottlenecked.
OP, I would wait for Skylake at this point. No games coming out this year will push your 2500K @ 4.2Ghz to warrant a worthwhile upgrade.