What happen if I buy a 750watts power supply?
Hi - There is no harm in having a 750w psu when you only need 500/550,
other than the additional cost initially.
The harm however is a poor quality PSU vs a good quality PSU
such as the XFX rolli linked. Be sure if you purch a PSU
of any wattage that it's a quality brand/model.
quality brands: Seasonic, XFX, PC P&Cooling, Rosewill Capstone series,
most Corsair's, most Antec's, Enermax, and a few others.