Summary: I'm an electronic tech and have designed and built power supplies for circuit boards and many devices. Instead of going into filtering and current supply, capacitors, chokes and heatsinks, I'll just say that this unit impresses me with it's 48 amps @ 12v and 87% efficiency. For most single video card systems this 620w model is plenty of power and I think you'll find that with even a GTX 780 it will most likely work.
Summary: Very nice psu! Pros. Modular, 24 pin and 8 pin board cables are hardwired. Seasonic build platform. Google is you friend here. Very quiet. Good length of cables, no need for extensions, in a mid tower case, length to spare. 5yr warranty. Complete w/paper work. Cons. No disappointments at all. Shipping. Arrived a day early. It was a rainy one. Shipping service had it in a plastic bag. WOW! That's super service. Once again, bought this here out of Amazon's Warehouse Deals.
Summary: I run 2 overclocked 7970s off of this power supply. Under load I pull 750w from the wall, this PSU runs flawlessly. I have seen power draw peak at 820w with no side effects, but it will shut off after more then a few minutes at that load. Under extreme load, this PSU is loud, but if you are not overloading it, it runs fairly quietly.
Pros: Powerful, will deliver more than rated watts., Feels solidly built., Working fine for a year and a half.
Cons: Stiff cables., Not many connectors, I needed some adapters to run my videocards and harddrives.