To get the specifications out of the way, the GeForce GTX 680M has 1344 CUDA cores, and runs at 720MHz (the clock is variable). The chip can address up to 4GB of video memory, which is accessed via a 256-bit bus with a maximum bandwidth of 115.2GB/s.
In terms of performance, things should be pretty nice too. Based on 3DMark 11, NVIDIA believes that this new chip is 80% faster than the GeForce GTX 580M, and that it will run many popular graphics-heavy games at 40FPS to 120FPS. For instance, NVIDIA’s performance numbers claim that Battlefield 3 can be run at 60FPS with the maximum details, PhysX enabled, in 1080p. By now, you can be sure that independent benchmarks against the AMD 7970M are popping left and right.
As usual with Kepler-based chips, you can expect to see things like TXAA anti-aliasing (for smooth edges), Adaptive V-sync (for suttering reduction), Optimus (shuts down the GPU to save battery). You can expect to see the GeForce GTX 680M in systems like the Alienware M17x and M18x or the MSI GT70.