if you currently have the A4-3300 then your motherboard is an FM1 socket which means the A8-3870K will fit since it also has an FM1 socket.
for the Graphics Card to work all you need is one PCI-E 1.0, 2.0, or 3.0 slot on the motherboard and a power supply with a high enough watt limit and the correct PCI-E connectors to power the GPU (at least one 6pin will due for most 7850 and less powerful GPUs but I prefer having two 6+2pins for versatility since they are pretty easy to get just in case I ever make a big upgrade in the future).
PCI-E 1.0, 2.0, or 3.0 is the longest slot in the picture here if you are not aware of what it exactly looks like. PCI | FinitPC
but according to the MB specs, it looks like you have one of each size that currently exists so no problem there.