Yes, your case will fit another GTX680 and your motherboard supports SLI for these cards. If terms of power requirements, a single card running in a complete system is recommended to have atleast a 550W power supply. Adding a second card will probably bring you to around the 750W mark. I would invest in a quality brand 750W PSU or 850W PSU if your budget allows for it.
9/17/12 at 11:36pm