I believe the A10 6800k can only crossfire with an HD 6670, although there are people claiming that it can go up to a 7750, which would be awesome but not officially recommended. If you crossfire your iGPU with an HD 6670, it should come out to about the same performance as a 7870 though, so if you don't want to deal with the shortcomings of the Crossfire system then just get a 7870 and use it as a discrete.
I've only ever done reading on the APUs and have never gotten my hands on any of them so I can't help you much beyond that, hopefully someone with more expertise can take over here