The GT 640M is better than the 7670M by a margin. But the 3rd Gen i3 is much much better than the 2nd gen i3 you have posted. I'd say to go with the 3rd gen i3 and the 7670M. It's better in the long run and will be satisfactory for SC2.
You don't have to take my word for it. Check benchmarks at:
Hope I helped!