I believe the 6570 is the closest discrete equivalent on the desktop to the top Llano APU. It would help to know your resolution, use for the card, power supply and budget for recommending a card. Do you only need something that matches Llano, or could you benefit from more GPU horsepower?
I am looking for a discrete GPU that has the closest specs to the onboard HD 6550. It needs to be as close to a perfect match as possible. I think it may be the HD 6570. Maybe I can down clock it to match speeds with the HD 6550.
I too have a A8-3850. The manual indicates that the HD6670 is the one to use and will be the one I buy. Several others also make reference to this on Newegg's user feedback for HD6670 GPUs. Up to you, but that is my recommendation. Go for the 1GB model with DDR3 for better matching.