2nd or 3rd line. The caveat being the 640M LE apparently has a Kepler version, and a Fermi version. One should know by now that Nvidia reserves the right to butcher any card name ending with an E (or without a Ti suffix if there's a Ti version).
If you're looking for a high-end card, 680M it is. AMD's 7950M and 7970M are going to be faster than anything Nvidia has right now, and most likely the 7970M will beat the 680M. But besides that, it should be close (slight, or perhaps big, advantage in favor of AMD).
You can find most of the info on the new cards here (check the sidebar to the right for 680M and 660M): http://www.notebookcheck.net/NVIDIA-GeForce-GT-650M.71887.0.html