I used nagios & cacti at several past positions, and was always annoyed at the need for two applications and also at the clunky (to put it mildly) configuration. This last time, when I needed to start monitoring systems, I did some research and discovered zabbix - it does the same as nagios and cacti, and it does it a lot better. All configuration is through the web, it has simple and very flexible notifications, and it has templates which means you can get a new system up and running with all checks in literally a minute or so (though admittedly it's a little longer if you need to install the client instead of just using SNMP).