On a slightly different angle, why not go for something like a 750W PSU and a GTX970 SLi. In terms of graphics cards you get way more GPU power for the same price.
As for your question, Nvidia state that the minimum power requirement is 500W:
Yup, the GTX 9xx series requires 500W PSU. It should work, but may push the limits. Like you mentioned though, a GOOD PSU at 550W is not the same as a junk PSU at 550W. So with a quality PSU, I see no problem...just cannot OC or SLI.