I'd have to vote for the Seasonic power supply. I still have my Seasonic PSU from four years ago churning away in my rig with no issues at all and a killer warranty (even though it's expired now I believe).
Pretty much this. Depending on your setup, with one graphics card most systems will draw 350-400w with CPU+GPU load with higher end systems like X79 possibly approaching 650w overclocked CPU+GPU.
If you plan on running two high end video cards you'll want 850w or higher whereas if you're running a system that doesn't draw much power you might be able to manage with a high end 650-700w PSU (core i5 not overclocked + two radeon 7770s for example).
On your average system the card requires you to have a 550 Watt power supply unit. We recommend at least 35 Amps in total available on the +12 volts rails (accumulated).
GeForce GTX 480
On your average system the card requires you to have a 600 Watt power supply unit. We recommend at least 40 Amps in total available on the +12 volts rails (accumulated).
GeForce GTX 400 in SLI
A second card requires you to add another 250 Watts. You need a 850+ Watt power supply unit if you use it in a high-end system (1 KiloWatt recommended). That power supply needs to have (in total accumulated) at least 55 to 60 Amps available on the +12 volt rails.