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.