What exactly do you want to monitor? On the host? (Iptraf, slurm, iftop, vnstat.. munin) On the switch? (mrtg, cacti, observium - via SNMP) On a software-based firewall? (iptables rules, iptables -nvL + scripting etc) On a router/switch that supports netflow?
Just monitor traffic usage per ip-address says nothing.
There are a number of flow exporters out there ntop is a complete package. It can work of a span port to your uplink or on each server exporting to a central flow reporter. This can be CPU intensive once you get past the 100mbs line.