I'm planning on a build with an AMD A10-5800K and have an old Sapphire HD 5570, I was wondering if anyone had any thoughts on how well crossfire would work. I mostly play Counter-strike: Global Offensive and the 5570 handles it pretty well on its own, but might look at other newer games.
i'm pretty sure the onboard gpu on the a10 is a better gpu then the old 5570. It also won't xfire/dual graphics with your 5570. You need a similar gpu core design, which means you'd need a 6670 to dual graphics with the a10.
As for xfire for two 5570, i think you'd probably enjoy the results of dual graphics with a 6670 on your a10 better.
The Radeon HD 5570 DDR3 or DDR5 are both better than the integrated 7660D on the A10-5800K.
The best discrete card you could crossfire with your 7660d APU is the HD 6670 as previously stated. Some games support crossfire and you see a strong benefit from it; others don't and you see literally no benefit and sometimes even a minute loss. If your HD 5570 is the GDDR5 version of the card, then it is very close in performance to the HD 6670 GDDR3 card. I bring this up because, if I'm not mistaken, the APU/ Discrete card crossfire does not support GDDR5 and subsequently runs at a GDDR3 speed. So my advice would be to just save up a bit more for a HD 7750 GDDR5 1GB. That card will play most games at medium/high settings very well at resolutions 1080p or lower and without the need for additional power from the PSU.