FYI, the 660m, 650m and 640m are all the same chip at different clocks.
In addition, the higher-end ones usually have the better GDDR5 RAM as compared to DDR3 RAM.
GW2 plays on quite modest machines on low.
Indeed even a pitiful Intel HD4000 will play GW2 on low.
The 640m and 650m (and 660m) should be enough to play reasonably, but you will not be able to turn everything up. (drivers have gotten better since laptopcheck tested)
Basic questing etc. is doable on a lower end cards if you consider ~20fps acceptable, however doing any sort of RvRvR is completely out of the question, even my sons 550Ti desktop card paired with an i3-2100 at 1600x900 resolutionon medium settings struggles to stay above 30fps (which is well above a 660M in performance) in big realm vs. realm battles.
I think it would disingenuous to suggest to someone that they would have a good experience in GW2 with a lower end card if they plan to participate in what most would consider to be the major selling point of the game, which is the large scale pvp battles.