A system with a single GTX560ti requires a 500W PSU with at least 36A on the 12V rails and two 75W 6-pin PCIe power connectors. The Seasonic M12II-520 is more than electrically sufficient with 40A on the 12V rail, has sufficient PCIe power connectors and is an outstanding PSU. I have one running an HD6950; does it quietly and with ease.
Edit: For some reason I saw the HCG-520 and thought Seasonic M12II-520. Probably because Seasonic makes the HCG 520 for Antec and it has the same base design as the M12II-520.
Seasonic M12II-520 is not available for me so i chose Antec.
HCG-520 has 40A at +12V rail. And has 2 PCI-e 6+2 pin. Is that compatible?
Also the card is factory oc'ed to 950MHz. Did you consider that too?