I wasn't able to find your 7640G on AMD.com, but I was able to find your 7670M, and from what I can tell here: Legacy - It seems that the 16.2.1 drivers are the ones you want. You also have the option of 15.7.1 (older non-Beta version), if you would like that one. As a side note, I've never heard of a laptop with two graphics cards in it. You may need to run through an uninstall to get the drivers to work properly.
If you feel you need to uninstall old drivers first, I would uninstall all AMD related drivers from the "Programs and Features" menu in Windows, then restart. Then use the AMD Clean Uninstall Utility, found here: AMD Clean Uninstall Utility - And restart again. This should clear your system of all old AMD driver software before you install anything new.
As a side note, I always uninstall the old drivers first before installing any new ones, and this is generally recommended.