I'm buying AMD A8 3850 and need advice for my old Graphics Card

Just put the 8600GT away in an antistatic bag for a rainy day. A working card is still good to keep around for emergency repairs to a friends / family members computer.

Try your new computer just with the onboard graphics. If you need more then the 6670 card is the fastest that will Hybrid Crossfire. These cards are $100 or less in the USA and cheap anywhere else. They make a big difference to performance and are easy to install and configure. They don't even need serperate power leads.