Arma 3 server hosting: why your CPU matters more than your RAM
Arma 3 differs from nearly every other game on this list: here your limit is not RAM but the processor. The game simulates AI units, ballistics and vehicle physics, and most of that work happens on a single core.
Clock speed beats core count
A server with four fast cores outperforms one with sixteen slow ones for Arma. That is the opposite of what you want in a web server. It also explains why a mission heavy with AI can stutter while memory use barely moves — you are hitting the CPU, not the RAM.
Headless clients
If you run large missions with a lot of AI, this is what most communities use: a headless client is a second connection that takes over AI calculations, lifting that load off the main server. For smaller missions it is unnecessary.
Mods and missions
Arma runs on mods — CBA, ACE and RHS are standard for many groups. Server and players need the same mods at the same versions; the launcher handles that through a preset. Missions themselves go into the mpmissions folder as .pbo files.
How much RAM
2 GB suits a small group, 4 GB for larger modded missions. But with Arma, watch the CPU above all: that is what you run into first.