Yes, your Seasonic S12II 520W is listed at tier 2 on the PSU tier list, and clearly high quality. Don't take my next statement as disrespect against your rig, but it makes me a little nervous that you're going under the recommendations. But since you're only running a low-end i5, it could actually work. If I were personally buying a reference cooler, I'd get (and I already have) the EVGA 980 Ti (reference). The reason is that their tech support is excellent; they have tech support desks dedicated to US, Europe and Asia. Also, since I have my EVGA products registered, my calls into tech support get the skip the line into a high priority queue. I call them sometimes with PSU questions. If I was getting a custom cooler card, I'd go for the MSI 980 Ti 6G Gaming card. Previously I owned the GTX 970 4G Gaming card and absolutely loved it. Whatever choice you go with, please let us know how it goes. Also, should you consider upgrading your PSU to future-proof (hate that term) your build, consider getting a tier 1 850 watt PSU. This will give you to option to install two 980 Ti's in SLI with up to a 5960X CPU. The best thing about PSU's is that you can take them to your next build. I have EVGA G2 850 units, and I really love them. Matter-of-fact, they just released a EVGA P2 850 and I plan on picking up one of those as well; the platinum efficiency is not needed for your build, so don't bother going above gold, unless you just want to. Good luck and don't forget to post a picture of the inside of your case once you get your new card installed.
GTX 980 Ti: 250 watt TDP
i5-3470: 77 watt TDP
Reference MSI: 750$
Reference Gigabyte: 770$
EVGA ACX 2.0+ SC: 780$
Reference Asus: 800$
EVGA ACX 2.0+ SC+: 825$
Your listed choices of cards really boils down to just two choices: 980 Ti (reference cooler) or EVGA ACX 2.0+ SC+. If you end up going with the custom PCB/cooler card then you could get just the SC (non +) and overclock the card yourself. Keep in mind that even cards that have two 8 pin connectors, such as MSI's 980 Ti 6G Gaming card will come with a single 6-to-8 pin adapter.