Ignore everyone telling you to OC... any decent processor and graphics card will run games fine without an OC. The only advantage overclockers see nowadays are in the 1-5 fps range on a good day with no wind and the moon a certain distance away (aka it's not worth it unless you enjoy doing it).
Nonsense.
I run a 120 hz panel and have an i5 3570k @ 4.2 Ghz steady coupled with a 780ti.
Want to know my bottleneck at 120 fps? CPU. That's right. 100% on one core and it hamstrings my fps minimum and average, and not by a little even.
If you aim for 60 fps tops, if you run a game that is 1 thread, you will also have situations where you are CPU limited.
The world of gaming is bigger than a few triple-A shooter releases buddy. When you build a system from scratch, today, with the knowledge that CPU advances in performance have begun stalling (because they have been ever since SandyBridge), and you put top end graphics in the system, overclocking is a no-brainer. It is worth the investment and a fast i5 is for gaming the ultimate cost-effective solution, spend the money you would spend on i7 Hyperthreading instead on better cooling and MB, and push the i5 K hard = better gaming performance at same price or less.
And the cherry on top; minimum frame rates benefit from a very fast single thread, even sub 60 fps this can be of influence. It benefits frame consistency. On quad core Intel, it is also managed out of the box in a way that it reduces multiplier at full load on all cores. You will notice that the max multi on a regular i5 will step back x1 or x2 when 3 or 4 cores are at work. An overclockable CPU allows you to remove that limitation. Even the worst binned K CPU has headroom on voltage as well, so underclocking at stock clocks is also an option. All beneficial to either performance or temps.
I have also seen people recommending an AIO instead of solid air cooling. Yeah. If you talk about nonsense investments, that there is a big one. An AIO either breaks the bank and gives you a few degrees C for it, or it is cheap junk at does a worse job than simple and cheap air cooling does. The big cooling issue today is different from what it was 10 years ago; with every node shrink, the concentration of heat becomes bigger and the surface to transfer and dissipate that heat becomes smaller. This is part of the reason why Ivy clocked less than Sandy Bridge (it wasn't just the TIM used).
