minecraft runs in java and therefor isn't multi-threaded. So in this case, the faster the clock speed, the better. It only needs two cores (the other to run typical background tasks), but it needs to be fast to track 20 people in different areas on the server. Next important part is reading and writing speed of your hard drive. I bought a small SATA3 solid state to reduce server lag for my game. As 20 people explore different area at the same time, your hd needs to constantly load new visible terrain and save any alterations people make to the terrain. You could always just create a RAM-disk for the fastest scenario, but I chose not to do it. Outside of that, buy the quietest components you can that are reliable, because 24/7 fan noise is annoying. This is all my own trial and error knowledge. Someone else might have more insight.