The best Discrete Graphics Card which can used in Dual Graphics with the A10-7870k is the R7 250.
The thing to remember about Dual Graphics (and CrossFire) is that both GPU's have to be running at the same speed for it to work correctly. This means that both cards will only be able to utilize the running power of the lowest end card. On a basic level this means that a higher end card has to "slow" itself down to match the speed of the lower end one. This is not something which can be overcome with "drivers" or "firmware updates" it is a requirement of the technology itself.
As such, when you check the specifications of the A10-7870k you will see that it only has up to 512 Stream Pipelines (Cores) and these are run at around 866MHz. Also APU's must utilize your Systems installed DRR3 RAM which is slower then "dedicated" GDDR5 RAM which a Discrete Graphics Card would use. Now if you were to use the Discrete Graphics Card R7 250 in Dual Graphics with the A10-7870k you would end up getting higher performance then only using one of those cards by itself. But if you were to use a R7 250X or higher card the overall performance of a single card would out-pace that same card run in "Dual Graphics" with the A10-7870k.
Also please remember that Dual Graphics (and CrossFire) only are enabled in compatible games, while playing in Full Screen Mode. As such a single card, higher then the R7 250, would be a better choice overall, if you are looking for performance. The HD 7950 has 1792 Stream Pipelines and normally comes with 2-3 GBs of GDDR5 memory. As such, this card by itself would far out perform any APU + Discrete Graphics Card setup.
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