Any kind of Overclocking will lead you to a warranty void is anything happens to the specific hardware component. Therefore, you cannot say that you can have a safe overclock, even without raising voltages. This is one aspect that most people aren't aware about when they are OC-ing.
Also, even raising core and memory frequencies will lead to higher temperatures because this means that the core and memory chips will work harder in order to provide more FPS in games (well the scientific story would be that raising the frequencies means that the electrons that cross the conductors will collide more often and produce more heat by Joule effect).
I would suggest keeping your default clocks if you don't really need OC-ing. I cannot think of one game that the G751JY/JT could not run at decent frame rates even in Very High/Ultra details (speaking of FullHD resolution).
But, if you do desire to OC your video card, then I would suggest using hardware monitoring softwares like HWMonitor and HWInfo in order to keep an eye on the temps after you OCed. Also, I would suggest using MSI Afterburner's OnScreenDisplay in games because this will allow you to check your temperatures while you are playing, without having to minimize de window (you know, those stats in the right/left upper corners from Gameplay videos on youtube).
As for the OC-ing part, I would suggest a stability test after changing the frequencies. I have seen people saying that they have a stable system with +135Mhz core and a 6000Mhz Memory (this means +1000Mhz from the default 5000Mhz) but I personally found that the most stable OC is +135Mhz and 5700Mhz Memory. I would suggest remaining at a moderate +120Mhz and 5500Mhz for now, as I don't know if the extra Mhz would make any difference. If you feel like spending some hours with stability testing then you could go for more than 5700Mhz for memory just to see where is the max for your card.
Either way, enjoy the gameplay!
