The 256mb 8600m gt is DDR2, while the 512mb version is DDR3...or something of that nature.
Either way, there is a fast and slow version of the 8600m gt. I don't know if ati has the same kind of setup, i'm not familiar with ati mobile gpu's. i just stick with the nvidia ones, they perform better.
That's actually backwards from what I have noticed. The lower amount is of the faster variety. I also don't know if ATI is similar or if the HD3650 comes in a 256mb variant. Since the 3650 is a little bit better than the 8600mGT to begin with, I would imagine the 512mb 3650 would be faster than the 256mb DDR3 8600mGT.