I have just recently did a build using an AMD A10-5800k apu and my friend I built it for would like a little more umph out of his new rig. I know you can run an apu in crossfire with another dedicated card but which ones are compatible with it and what is the highest end option. I have been reading some forums but would like to clarify that the 6670 is the best card for the job. Also is it true the GDDR5 version of the card will be nerfed to GDDR3 memory? Thanks for any help on this one!
I believe you have to use a GPU with DDR3 memory only. It may not work at all with GDDR5, but you'll have to research that a little more to be sure.
You might be able to use an HD 7750 as that was the highest GPU you can crossfire with the A10-6800K, but I don't know if it works with the 5800K. Again, you'll have to do some more digging to find out.
Running crossfire with an APU will boost performance, but just understand it's not the same as crossfire with two discreet GPUs. It won't double your performance, but you might see respectable gains, depending on the game being run.
Of course the ideal solution is to just run a single, more powerful GPU, like an R7-260X or better. But if your budget is really tight, running crossfire with the APU is a viable option. Just don't expect to be blown away. ;)