the 7670 is a 6670 so it really is the top card you can use.
the 6670 and the the APU works well together if the drivers work well, they don't work well in some games tho.
you are correct that a 7670 is just a rebranded 6670 but good luck finding a 7670 for a desktop. They seem to be localized to the laptop market. You can find plenty of good deals on 6670's though.