Only overclock if your temperatures are good.
If you have Battlefield 4, play the multiplayer on that at ultra settings, put your core clock up by 10Mhz and play for a while, if it doesn't crash, its probably stable, continue going up 10Mhz at a time until your game crashes or you get artifacts on screen, then put it back down 10Mhz and test it for longer.
Battlefield 4 is good at sniffing out unstable overclocks.