Depending, you might want to get into full-blown network monitoring. This might seem like overkill, but it's really not as bad as it sounds:
find out if your system is running or could run an SNMP daemon and then use MRTG for network data collection and graphing. If you have no SNMP daemon on the server, you might also be able to SNMP query the network router or switch instead of the server to get the traffic in and out of the server's interface.