Lag :
I myself have gotten to a stable 3.7 ghz stable OC without upping the voltage (1.35v). But you really need to experiment with it, as every chip is different, and not everyone can reach the same clocks with the same settings. BUT, never go above 1.4v, as this is a sure way to seriously degrade and damage your chip. If you OC, the 1,4v should be your absolute max limit, as anything above that will just create unnecessary heat. Try to OC to 3,7 ghz first, and see if it stable. When it is, try to lower your voltage as much as you can (till its stable), and keep it there.
How do I check if it's stable?
I just ran prime95 for like 2min and my cpu cores reached to 65c lol. Turned it off.
But that's probably because my fan sucks and my thermal paste is dry as fuck.
Yes, ideal temperature for this cpu is about 55-60 if you want to keep it for a while, because max safe temp is 62 degrees. I would apply a new coat of thermal paste, which can dramatically reduce temps depending on how dry the old is. clean off all the thermal paste using rubber alcohol, and apply a small grain sized dot on the middle of the cpu, and let the pressure from the cooler do the rest
If your temps are still high, reduce the clock to 3,6, assuming you tested with 3.7. As for how to check if the overclock is stable: if you don't get BSOD, or the computer starts acting strangely, then the OC is generally stable. But temps is also important, and as i said, 55-60 degrees should be the max temps to aim for