Srry,it's 625w extreme 2,nt 665wWhat exact model of the power supply? and for my knowledge i never saw a 665W power supply.
Ok I will try..to buy a new PSU..Well the power supply you have is a really poor quality power supply.
Before buying a new gpu you should consider buying a new good power supply instead.
Ok ..which PSU I should buy?Well yeah it is working fine in the system, but it is not a good quality one, plus to begin with you didn't even have a graphic card and now you are adding one, that power supply wont be enough to supply enough power which can fry your components.
And before buying another power supply ask, some of them are cheap knockoff that you shouldn't buy for a rig.