Yes, he can get by with 750W easy. I run a 1000W with 3 GPUs all OC with a decent OC on my CPU as well.
No, there is no overvolting. All ATX put out the same voltage on the same pins. The only difference from PSU to PSU is current capacity and efficiency (well build quality too).
Your machine well not draw more current than it needs so using 1200W on a machine that pulls 200W will not cause damage, just a waste of money.
Having more wattage available than you need (head room) will allow the PSU to run cooler and therefore usually quieter.
I typically buy a PSU that has head room so it does run cool and silent and gives me the option where I can expand my system with no worry of buying another PSU.