I have a Seasonic 620W 80+ certified for an older backup gaming rig that was the main gamer at one time (overclocked E8400 -> 4.4GHz, GTX 275 SLI). It has absolutely been flawless with all those components. As others have referenced, Seasonic's design is actually used by other PS vendors like Corsair, OCZ, and XFX. Go with the Seasonic.
You may want to look into spending a little more and get a higher wattage supply for future upgrade options. The 12v single rail number is important to keep an eye on for certain video card requirements. My original PS for the above backup gaming rig was a Corsair TX850 purchased over three years ago. While overkill at the time, and a little more costly than the 600-750W I could have gotten by with, it's now in my current gaming rig running fine and still has headroom (overclocked 2500k -> 4.83GHz, GTX 680, soon to be SLI when I can get another).
Finally, power supplies do degrade in performance over time and lose efficiency, but the higher quality you buy up front, the less this degradation will occur over time.