Level 50
Munin, Zabbix and Prometheus are quite popular. Depends what you're wanting to 'monitor' though - you could end up with something using logstash and kibana. If you're just after the basics Apache has 'mod_status' which you can turn on that gives you this kind of thing. I'd assumed nginx has something similar.
You're not short of options really depending how much insight & details you want and the scale of what you're doing :-)