AMD Radeon R9 390x.

Solution
Depends mainly on the PSU's quality. Unless you have an obscene configuration, a good quality 550W PSU would be more than enough. 80+ "certified" means next to absolutely nothing quality-wise.


It's the efficiency:
http://www.hardwaresecrets.com/understanding-the-80-plus-certification/

It's possible to get a poor quality PSU with even a gold rating.