No it will be fine. Its better to future proof your system so u dont have to buy another PSU when you decide to upgrade later down the road with SLI. Same thing im doing with my GTX 590. BTW Corsair makes great PSUs.
No problem with doing that. The 750W indicator only means that the maximum capacity of the PSU is 750W but it doesn't necessarily mean that it always pumps out that much power to your components. Just like when your computer goes into a low power state. The components pull off only a small amount of power from your PSU and so that's the amount of power your PSU provides.