As expected, even with a 100MHz increase over the older AMD A8-3850, at stock speeds the A8-3870K doesn't distinguish itself fully against the Core i3.
Put the two chips head to head solus and it has the edge, but if you're using external graphics the Core i3 still pulls away in any game that's not massively multithreaded (Shogun 2, tellingly, is).
And what of the overclocking? It's actually a bit disappointing really. At the speeds we achieved using fairly basic equipment, the difference is often not enough to be worth even trying.
DirectX 11 gaming performance
DirectX 11 tessellation performance
CPU rendering performance
CPU video encode performance