If you want to take full advantage of the onboard 7660D and combine it with a discrete graphics card take a look at page 27 for the fm2a85x extreme 6 guide. Note for step 2 below. Since AMD does not make a 7450-7670 use a 6450-6670 GPU. There is no mention of the 7850.
I hope this helps you to stick to the discrete 7850 only.
2.6 AMD Dual Graphics Operation Guide
Currently, AMD Dual GraphicsTechnology is only supported with Windows® 8 / 7 OS, and is not available with Windows® VistaTM OS.
Step 1. Please keep the default UEFI setting of “Dual Graphics“ option on [Auto].
Step 2. Install one AMD RADEONHD7450/7470/7570/7670 PCI Express graphics card to PCIE2 slot.
Step 3. Connect the monitor cable to the onboard VGA port. Please be noted that the current VGA driver /VBIOS can allow Dual Graphics output from on- board display only.
Step 4. Boot into OS. Please remove the AMD driver if you have any VGA driver installed in your system
Step 5. Install the onboard VGA driver from our support CD to your system for both the onboard VGA and the discrete graphics card.
Step 6. Restart your computer. Right-click the desktop. Click “AMD VISION Engine Control Center” to enter AMD VISION Engine Control Center.