Is 16GB RAM overkill for a budget i3-12100 build?

No, 16GB is the new baseline — not overkill. 8GB is still technically enough for light browsing and old games, but Windows alone uses 4GB before you open a browser, and adding tabs, Discord, and a modern game will cause swapping to disk. The i3-12100 is a solid CPU, so don’t bottleneck it with skimpy RAM. RAM is cheap (around $30–40 for 16GB DDR4) and future-proofing here is a no-brainer.