I've been seriously considering a new build based on the 3400G.
I also hardly game anymore, the odd session with Diablo 3 is about my limit these days.
My reason for considering the 3400G are:
Existing machine (Xeon 1230v2 - 3.3 to 3.7GHz) is getting old - I'm unwilling to consider a replacement processor with inferior specifications so - I'd like at least four cores with 8 threads, higher clockspeed and lower power consumption. The downsides are the reduced level 3 cache 6GB vs 8GB on the Xeon. The Vega 11 processor offers inferior graphics pefrormance to the Nvidia 750ti in my current machine.
As RavenXXX2 says, pound for pound, the 3200G is better value at around two thirds the price. For a bit more the Ryzen 5 3600 is still great value without the graphics core.
Consider your current graphics card (assuming here you'll reuse components) if the onboard graphics on the 3200G/3400G are quicker that your existing graphics card, then it's a matter of how you budget.
If the onboard graphics on these chips are slower than an existing card you have, I'd think very seriously about the Ryzen 5 3600.
Lots of other factors of course. Size of case, noise levels, motherboard etc. Looking one processor down the food chain and one up is a good way to start budgeting a build.
Regards,
B.