750 or 650?
Generally, when I build a PC, I like to get a PSU at least 100 watts above what PCPP says. Realistically you don't need more than a 550 watt PSU. So if you're dead set on 750 or 650, go for the 650.
Definitely go for the 750, it's better to be safe and prepared for future upgrades you might make.
550 watts is plenty for the build as is and for SLI a 650 watt or a 750 watt would be enough.