The 675M is a rebranded (renamed and re released) 580M according to notebookreview. It is the same chip with identical clocks.
The 680M specs have not yet been released, and as of a few days ago early speculation indicates that it may not be a true kepler chip, therefore a bit of a disappointment.
New rumours suggest that it may indeed be a kepler chip.
and 670m is rebranded 570m... 660m is the only GTX keppler chip so far and it'll come with a 128-bit bus which will only allow it to be slightly faster than gtx 560m.
Hence why I didn't wait for the new generation but bought my lappy with gtx 560m and will consider the GTX680m and corresponding amd radeon when my 560m stops being enough for me. (I'm running it at higher clocks so it's performance is around gtx 570m/670m stock)