In my experience, most games use between 14 - 18GB of DRAM with the settings I mentioned, which is why I recommend 24GB.
A graphics card with 12GB of VRAM is sufficient for current games, as they typically use between 8 - 10GB, especially in XBOX Mode, which reduces RAM usage.
With this combination, you get a next-gen experience comparable to current consoles at a relatively reasonable price.