Hello, I have some question. Now I have A10 7850K and R7 250, which are working as dual graphics mode. But how can I upgrade my PC's graphics and still have dual graphics mode? Will A10 7850K + R7 260 (without 'x'), or any other video card provide dual graphics mode?
Dual Graphics is -extremely- weak, the R7 260 alone is more powerful than your current setup. No current or previous generation card supports Dual Graphics due to the fact that the supported models are so weak. You're much better off getting something like the RX 460, which is near 200% faster than your current setup.