how does having 2GB of graphics enhance my graphics experience?
The 640M isn't really that fast utilize 2GB. So go for 6xxx or 7xxx series in AMD, Because AMD series is the best one to utilize 2GB.
2gb won't get fully used by 640m level cards. But @JuGGa you need not have quoted the gtx670 . That's a baap. It will take most games a hell lot of pain to even to use up 1gb on that card. Multi-monitor full hd setup and some helluva games like Alan Wake or Just Cause 2 will be the recipe for upper mid level cards to get stressed to the hilt (and that does not mean the memory).