A system running a single GeForce GTX 570 graphics card requires that the power supply have a combined +12 Volt continuous current rating of 38 Amps or greater and have at least two 6-pin PCI Express supplementary power connectors.
The Seasonic S12II-520 Bronze does not have two 12V rails at all - it's a single 12V rail design. There is no multirail overcurrent protection in there. It has a +12 Volt continuous current rating of 40 Amps which meets the requirement to run a single GeForce GTX 570 graphics card system.
The GeForce GTX 570 itself can draw up to 298 Watts (i.e. up to 24.83 Amps from the +12V rail(s)) when running FurMark. FurMark isn't considered a normal power draw because you'll only encouter this type of power use when running GPU torture testing utilities (e.g. FurMark, OCCT GPU Torture Test). A more realistic power draw, like that encountered during intense gaming, will be up to 190 Watts (i.e. up to 15.8 Amps from the +12V rail(s)). Overclocking the graphics card, will of course, increase the power consumption.