The Ryzen 5 2600 paired with a GTX 1660 Ti on a B450 MB with 3000 Mhz or 3200 Mhz RAM is a good mid level build at a reasonable price.
I upgraded my rig in November to:
Asus ROG Strix B450-F Gaming
Ryzen 5 2600 (which I can OC to 4 GHz)
Arctic 33 Esports Edition cooler
16 GB GSkill TridentZ 3200 F4-3200C16D-16GTZR (Which I run at 3400 MHz)
That cost me just over $400.
I recently upgraded from my GTX 1060 to an MSI GTX 1660 Ti Armor OC for just under $300. That made a Huge difference, I was gaming at 1080 at 60 fps with medium settings, now I game at 1080 at 85-120 fps on Ultra settings.
If you don't want to mess with overclocking, the Ryzen 5 2600X is a good option but costs $40-$50 more.
Also the MSI B450 Tomahawk or Gigabyte B450 Aorus Pro also seem to be decent options for a MB in the same price range.