If it is absolutely stupid graphics, then MRTG with SNMP (if the load is not needed, without SNMP, the usual popinga enough). Here, for example.
If a little more serious, then Nagios. Habré ton of articles.
Zabbix is just here that the offices of intelligence, a lot of graphs and stuff :) I would not recommend it, especially under a single server.
Munin itself did not use, but judging by what you tell me is also a good fit.