Monitorix is the monitoring tool of my choice (though I'm running Debian and Ubuntu on my machines; Monitorix is available for many distributions, including RedHat/Fedora/CentOS). It of course includes network traffic...
Network traffic: overall and per port (click images for original size)
...but also much more system health criteria, even details for several network applications (web servers/caches, bind, MySQL, etc.). Naturally, disk usage, load, and overall health state are included as well. Saved my day more than once. Checkout the screenshot page for more details.
system load and file-system usage
Monitorix is light-weight, includes its own web service (by default, listening on port 8080) which can be protected with username/passwort (HTTP Auth), and is well maintained (disclosure: I'm its maintainer for Debian/Ubuntu packages and the corresponding repository). 24/7 you find the dev himself and several of us maintainers on its IRC channel if you have questions.