670m is a pile of junk compared to a 650m. I personally think a 650m is an amazing card. 650m is basically replaced the fermi gt540m. Because of fermi parts in the 600m series it had to be named a high 650m when its just a low profile card designed for ultraportables. This gpu is as powerful as a desktop hd 5750/5770 when oc'd.
In essence a 650m takes around 30-35w power consumption wise. The 670m takes around 100-110w in actual power consumption. Reviews show the 680m consumes around a few watt less then the 670m and the 680m is over double the speed of the 670m. To be honest 670m is an ancient card, recycled and now is just designed for nvidia to make money with. Its like nvidia selling a gt 650m in 3 years time with maybe slight oc and selling it. By then new gen maxwell is out.
Either way why do you want to upgrade? 650m is plenty.