Your graphics card is efficient and will run on as little as 430W.
I have no problem overprovisioning a PSU a bit. Say 20%.
It will run cooler, quieter, and more efficiently in the middle third of it's range.
A PSU will only use the wattage demanded of it, regardless of it's max capability.
I might look at 550w or so to so you have power to upgrade the graphics card to something as good as a GTX1080.
DO NOT buy a cheap psu.
A cheap PSU will be made of substandard components. It will not have safety and overload protections.
If it fails under load, it can destroy anything it is connected to.
It will deliver advertised power only at room temperatures, not at higher temperatures found when installed in a case.
The wattage will be delivered on the 3 and 5v rails, not on the 12v rails where modern parts
like the CPU and Graphics cards need it. What power is delivered may fluctuate and cause instability
issues that are hard to diagnose.
The fan will need to spin up higher to cool it, making it noisy.
A cheap PSU can become very expensive. Do not buy one.
Seasonic will be a safe purchase.
Here is a quality list, there are others.
http://www.tomshardware.com/forum/id-2547993/psu-tier-list.html