monitoring system resources for localhost?
 

i've been surfing around looking at the different tools to monitor my redhat system and i am confused. There is nagios, snmp, mrtg, rrdtool. They all seem to have plugins for each other. What is the difference among them?

I am looking for something that will give me graphs over a period of time. For instance, when during the day are resources peaking. Did i have a spike in resource usage. I think no matter what snmp needs to be installed and configured. I am only monitoring this one machine. Is there a security risk with snmp? is all good if i block the ingress and egress port and have it only listening to the local interface? What other security steps would i need?