The 2GB HD6670 is almost completely pointless, and here's why...
A GPU benefits from additional memory over 1GB when you use high resolution textures, game at a high resolution and/or use high in-game settings like 8xAA for example.
However the HD6670 doesn't have enough processing power to deal with the demands of high resolution gaming or high in-game settings. (even when paired with your APU in crossfire).
So the frame rate will be so low before the extra 1GB of memory even gets used at all that its pointless having it.
Get the HD6670 1GB GDDR5.