This portion of the presentation will assume debian package names and configuration
On the server (collector):
Need a web server
Need nagios, nagios-plugins, a database backend (e.g. nagios-text), and, if using nagios-nrpe, nagios-nrpe-plugin
For host monitoring the clients to be monitored need one or more of nagios-nrpe-server, nagios-plugins, and nagios-statd-server