I have the same APU/GPU combo on my Lenovo 585. The 512MB refers to the memory allocated to your APU's gfx (7640G), shared from your system RAM. It does not include the dedicated memory from your discrete GPU (7670M), which from what you wrote is most likely 1GB. From what I've encountered, display info for 7640G+7670M only refers to the specs of the 7640G.
Sandra Lite (in the Displays and Video Adapters section) reports 2.5GB gfx memory available for DirectX 10/11 on my machine, since my dedicated card's memory is 2GB.
GPU-Z is another free utility to list useful info for your gfx devices and has a drop down selection box to change between the two, but it behaves erratically on my laptop, freezing at the splash screen when 7670M is selected from a previous session (unless, oddly enough, Sandra Lite is already running, or is run during the splash screen hang up and Displays and Video Adapters is selected) and also misreports the amount of available RAM for the 7670M as an insanely large amount (again unless Sandra Lite is already running, then it correctly displays 2GB).