It depends on the game but a RX 570 would be fine to pair with that 6100 and if your only using the iGPU on the 6100 then your going to see a massive improvement in performance/graphic settings.
Currently I have a G5400 which is very similar to your 6100 paired with a RX 580 and only on more CPU intensive titles do I have to lower the settings to keep the FPS within the Freesync range. On other games though I'm able to up the graphics to high/ultra and keep the FPS at 60+.