Dual graphics on a AMD A8-3870k?!?!?

There is no point of CossfireX that GPU. Its too weak.
Better to get single GPU. Like GTX 650 Ti or better.

I was going to use Dual graphics, I understand nothing you just said explain more please.

By the Above Explanation, You are saying that you will Buy 2 Radeon 6670 for your Computer. Right?
You'll Crossfire two 6670.

No, Im crossfire the AMD Apu with the 6670.

Yes, it can, but only the DDR3 one, although, there's like the poor driver support for that and low performance and micro-stuttering, i'd just get a 7750 it will be better than the 6670+apu(it could be better in some games when it's supported, but at cost of stuttering).

If you add anything higher than the 66XX series, it will disable the on board and use the added GPU.
Crossfire will only work with at most a Radeon HD 5670 or HD 6670.
Any Radeon card faster than one of those two will automatically disable Crossfire.

So, 6670 is the Best for your CPU.

Ok, thats what I needed also, but I asked will ANY 6670 Work in Crossfire with the CPU

Yup.