You buy a APU for the excellent integrated graphics.
But, there are no real good upgrades, particularly for a gamer.
If you install a superior discrete graphics card, you will have thrown away the big advantage of the APU.
Then, you are left with a relatively weak cpu. Most games depend on only a few fast cores.
The possible upgrades are to more cores, but few games will use more thn 2-3 cores so 6+ cores are not very helpful.
Bottom line.....
What you get with a APU is what you will live with forever.
Your current A6-5400K is a dual core with a passmark rating of 2137/1324(total/single thread)
The A10-7870K is a quad core with a passmark of 5501/1462.
A nice upgrade if your games can use more than two threads, but the single thread capability is largely unchanged.
Single thread performance is important for games such as sims, strategy and mmo types which tend to be cpu limited and single threaded.
If you change the motherboard, a FX-6300 is not much better. 6341/1409.
Good for multi threaded apps, but not so much for gaming.
The i3-4170 has 4 threads and a rating of 5176/2142.
It is a very nice upgrade but you will need a lga1150 motherboard. Even the cheapest will do.
The nice thing about the lga1150 path is that you have upgrade options to i5 and i7.