Need Help for OC'ing GTX 680M

Hi, I'm just learning all these things about overclocking and it would be very much kind of you to help me. I just have some simple queries.

These are my Specs:
Alienware M17X R4
Core 17 3740QM
GTX 680M

After reading all sorts of forums from here and there, I think the basic thing we are performing is to increase the Core Clock of any GPU, thereby pushing it to its limits at the same time keeping its temperature in check. Please correct me if I'm wrong.
Also, Overclocking can't be done with the GPU's vBIOS locked, thereby we must first unlock it by flashing it.
Now that the Vbios is unlocked we can adjust its clocks and voltage levels. I hope I got it right till now.

I use MSI Afterburner to monitor and overclocking (At least I thought it was). I can increase my GPU's core clock without flashing any vBIOS.
I get a stable 718 MHz at 2D and around 757 MHz after enabling 3D. Won't I get a better performance just by increasing it's Core Clock speed?
Also, regarding the voltage levels, what do they mean and do we have anything to do with it while overclocking?

I was so confused after reading forums from almost everywhere, maybe that was my mistake. Maybe I believe half-knowledge is poison.
I may have got things wrong above, please correct me.

Awaiting Replies.