What's your budget, what laptop are you looking at, and what games do you intend to play?
I own a Sony Vaio with an Nvidia Geforce GT 640M LE, which is a step below the GT 640M, but I'm very satisfied with its performance (and I've heard it easily overclocks to be almost comparable to the 650M). I get decent framerates on BF3 (around 40 average), and very good performance on most other games. I would definitely recommend a 640M, seeing as it's a step above.
However, if your budget's a little higher, there are some much better gaming laptops.