A APU is a decent buy so long as your expectations are modest and you have no plans to upgrade.
The integrated graphics is quite good, but once you install a discrete graphics card you have negated much of the value of an apu and are left wit a processor that has slow cores.
Many games depend on the performance of a single master thread. Few can use more than 2-3 threads. Often you will see 50% activity on both cores and think you pave plenty of cpu capability left. This is usually an indication of a cpu limited game that is single threaded. The activity you are seeing is windows alternating activity across available threads.
Games such as strategy, sims, and mmo are in this category.
There are a couple of experiments you can do:
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To help clarify your CPU/GPU options, run these two tests:
a) Run YOUR games, but lower your resolution and eye candy.
If your FPS increases, it indicates that your cpu is strong enough to drive a better graphics configuration.
If your FPS stays the same, you are likely more cpu limited.
b) Limit your cpu, either by reducing the OC, or, in windows power management, limit the maximum cpu% to something like 70%.
Go to control panel/power options/change plan settings/change advanced power settings/processor power management/maximum processor state/
This will simulate what a lack of cpu power will do.
Conversely what a 30% improvement in core speed might do.
You should also experiment with removing one core. You can do this in the windows msconfig boot advanced options option. You will need to reboot for the change to take effect. Set the number of processors to less than you have.
This will tell you how sensitive your games are to the benefits of many cores.
If your FPS drops significantly, it is an indicator that your cpu is the limiting factor, and a cpu upgrade is in order.
It is possible that both tests are positive, indicating that you have a well balanced system, and both cpu and gpu need to be upgraded to get better gaming FPS.
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The passmark rating of a A6-6400 is 2281 and the single thread rating is 1412.
The single thread ratings of most amd processors is in the 1500 range.
By comparison, the passmark rating of the cheapest $60 G4400 skylake processor is 3703 and the single thread rating is 1869.
If you decide to change your cpu, I might suggest a i3-6100 (5500/2102)
You will need a lga1151 motherboard and ddr4 ram.