Any card you get that is more powerful than a 6670 will downclock itself to imitate the performance of a 6670, before it can crossfire with the A10 5800k
So getting anything better is a waste of money.
You will be better off getting a single 7870 OC and run it without crossfire, as it will outperform the 6670 in crossfire with your A10.
you don't know what you're talking about.
nothing will downclock when put into a mb with an APU. if you try to "dual graphics" a non6670 with an A10/A8 nothing will happen. Dual graphics works just like xfire, in that you need a similar gpu core to sync up to.
A 7850 placed into a mb with an APU will work like a 7850 in ANY other motherboard. if you try to dual graphics it with the APU nothing will happen.
As to the OPs question, the 7850 or 7870 by themselves are way better then an APU in dual graphics with a 6670