ZOTAC's GeForce GTX 680 Extreme Edition is shaping up to be a monster. A preview by Expreview points out its clock speeds out of the box to be 1.20 GHz (base and boost), with memory staying at 1.50 GHz (6.00 GHz GDDR5 effective). The default core voltage is 1.212V. ZOTAC also told the source that the card is capable of delivering 1.5V core voltage to assist in extreme-overclocking. It also goes on to claim that 2.00 GHz core clock speed might just be possible, in the best case scenario (you have the best cooling, best overclocking skillset, best luck).
Apart from a strong VRM (detailed in the older article), the card is also backed by several nifty features, such voltage measurement points, extreme-cooling optimized BIOS, and adequate-looking VRM cooling. The USB mini type-B connector on the back of the card turns out to be not a connection to an OC module, but a USB connection to the motherboard. ZOTAC is routing the card's SMBUS to its Firestorm OC software over USB. Maybe they haven't figured out [how] this could be done through the NVIDIA driver interface, or they didn't trust NVIDIA. For example, NVIDIA blocks SMBUS access to components that they deem "sensitive".