Hello pachonk, my GPU is a 7640G as well, I´m using -v -w128 as my flags and I pull about 40-47 Mhash/s.
I really doubt we can pull anymore than that since it's a mobile GPU, but I would also like to know if it's possible to get a little more juice.

http://gpuz.techpowerup.com/13/04/22/a3b.png