Definitely a *huge* upgrade in CPU department. See for yourself:
https://cpu.userbenchmark.com/Compare/Intel-Core-i7-2600K-vs-Intel-Core-i5-2400/621vs803
In short, at stock clocks, you are looking at 20% better single core speed, a huge 60% when all cores are used, and both figures increase to 35% and 79% respectively with a good overclock.
Even without overclocking it is a nice upgrade, especially for newer games like BF1 / BF5 which tend to use all cores they can get.
With OC, you won't even see much difference from 7th gen Intels in gaming.
A good deal, although I'd try to lower that price by $10 or $20 just to see if it is possible
Which motherboard do you have, btw? And which CPU cooler?