Hexxit is a fairly light pack of mods. So you'll be able to play without any issues on a 2GB RAM server if you're gonna be playing by yourself. However, we'd recommend at least 4GB RAM if up to 5 friends will be joining you. For up to 15-20 people, we'd recommend at least 8GB RAM, however, 12-16GB RAM would be much smoother and lag free. General rule of thumb when calculating RAM usage is allocating 600 MB for every player.