So your options are;
A8-3850 as the highest APU you can drop in.
Due to an A55 chipset you can drop in 2x8GB DDR3-1600MHz ram sticks(a kit would be a good idea).
You can drop in a discrete GPU liek a GTX1050Ti or of their ilk but the PSU will be your limiter.
An aftermarket reliably built ATX 450W PSU will be needed apart from the above.
I would not spend money on anything that can not be carried forward to a future build.
Your psu will limit the possible graphics upgrades to a card that needs no aux psu power.
The strongest might be a GTX1650 or GTX1050ti.
That will do you some good if you play fast action games.
If you play cpu intensive games like sims, strategy and mmo then your cpu will be the limiter.
You could replace the processor, but you are not looking at much of a gain.
While it will not particularly help with gaming, (perhaps on level loads) you could replace the HDD with a ssd.
It will make everything so much quicker.
Such an upgrade will not be wasted if you upgrade to a new build later.
A samsung 1tb sata ssd is in the $110-$135 range.
Think 860 QVO @$110.
The conversion is trivial using the samsung ssd migration app.
Such a device can also be carried forward to a new build.
More ram might be helpful, but no current motherboards use ddr3 any more.