There's not much of a straightforward answer when it comes to questions like these.
I'm looking for something with the same or higher clock frequency as the Radeon HD 8570D [800MHz] that's integrated onto the APU.
And maybe Eyefinity support.
AMD does not provide a list of recommended graphics cards for A8-6800b on their website (outside of mobile variants). However, boxed APUs come with dual-graphics support information, printed on the side of the boxes. It should state that the HD 6570 and the HD 6670 are compatible with the HD 8570D for dual graphics (integrated GPU on the A8-6500). The HD 6670 has a core clock of 800mhz and should work well. Both the HD 6570 and the HD 6670 have Eyefinity Technology. You can check the product page of each GPU for the Eyefinity requirements.
I know the Radeon R5 230 is CrossFire capable with the Radeon HD 8570D. I've tried it before and it worked.
I've tried it with the Radeon HD 7770, but that combo doesn't seem to work and instead I can only use one or the other despite having both powered on and enabled.
Just wished there was an extensive and updated list for all of these...
Here's a list from AMD DUAL GRAPHIC webpage showing all the compatible AMD Cards that will work in Crossfire with APUs: Radeon™ Dual Graphics Cards | AMD.
Your APU is not listed.
From CPU World.com concerning Crossfire on your APU: http://www.cpu-world.com/info/AMD/Recommended_graphics_cards_for_AMD_dual-graphics.html
AMD does not provide a list of recommended graphics cards for Ax-6xxx desktop products on their website. However, boxed APUs come with dual-graphics support information, printed on the side of the boxes. It shows the following compatible graphics cards on the package:
Update: With the only exception of HD 8470D and HD 6570 combination, AMD confirmed these recommendations. The HD 6570 GPU was shown as supported on AMD boxed processors therefore we left it in the list.
Just confirming seahawkshunt post with a visual.
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