Good quality 600W range unit will do fine.
As far as Seasonic X-650 vs Seasonic S12II-620 goes, both units have good and bad features. X-650 is great quality PSU while S12II-620 is good quality PSU.
X-series PSUs have somewhat noisy fan due to the double ball bearing. But it's 80+ Gold certification, fully modular cables, switch for Hybrid Silent fan control and 7 years warranty are great features.
S12II series PSUs are fully wired and they will give you great headache in cable management. Though, it's 80+ Bronze certification, fluid dynamic bearing fan and 5 years of warranty can outweigh fully wired design. Not to mention cheaper price than X-series.
If i'd have the pick, i'd go with X-650 just because of fully modular cables and Hybrid Silent fan control the reduce fan noise under loads. Though, if you don't have money for X series then S12II is also a great choice, just don't mind the fully wired design.
All my 3 PCs: Skylake, Haswell and AMD are also powered by Seasonic, full specs with pics in my sig.
I also have S12II series PSU in my AMD build but mine is 520W. PSU has very solid build quality and it even powered my Skylake build for few weeks before i bought new PSU for it. All my Seasonic PSUs have served me well without any issues.