Be aware that FAN NOISE in a Power Supply is usually constant until about 50% load then ramps up. With a large CPU overclock it's likely that under load you'll cross over 50% for the 620W.
Therefore there's a chance that the 750W would be quieter under load. The price difference is small so I recommend the 750W.
In case you didn't know, the average upper STABLE limit for overclocking this CPU is about 4.6GHz (with a good heatsink and motherboard).
Also, when it comes to gaming the overclock will rarely benefit you with the GTX670. It's pointless in general if:
a) you already get 60FPS (you'll want to synch to 60FPS probably, likely with Adaptive VSYNC)
b) the game is more limited by the GPU (most games)
Just FYI. I have the i7-3770K which can auto TURBO up to 3.9GHz which as the DEFAULT also gives me the advantage of the best power managment settings so my PC runs cooler. I've seen no real need to overclock in any game.