Soldato
I'd probably go 850w, it gives you a bit of extra room if you decide to upgrade the gpu etc and even if not it will likely be in the sweet spot for efficiency and no fan mode if it has it.Think it would be worth getting a new 650W PSU? Or future proof a touch and go for 750/850w?
You could contact EVGA and ask them if they have any reference/info on the psu age (I had to upload my receipt for my gpu when I purchased it for example) or if they're prepared to go via the serial or something... worth a shot even if you buy a new one.