the wattage is *just* enough but it does not have two 8 pin PCI power connections for the gtx680.
edit: ooppps i was checking this review:Gigabyte GeForce GTX 680 SOC review
Installation of any of GeForce graphics cards is really easy. Once the card is installed and seated into the PC, we connect the two 8-pin PEG power connectors to the graphics card.
your card may be different, a more "vanilla" design with two 6 pin connections, which then the 520 will work.
Here is Guru3D's power supply recommendation:
GeForce GTX 680 - On your average system the card requires you to have a 550 Watt power supply unit.
the 520 is close enough given its quality . . .