The standard Nagios plugins should include an SNMP plugin that can check the bandwidth usage of a NIC just fine. I've never bothered to use WMI when SNMP was much more interoperable in a mixed environment. I try not to use NRPE or NSClient as it means more work to maintain the plugins if you make a change or update them.
As for being able to use RRD tools to graph the data from Nagios, you will have to setup a script that will collect the performance data from the Nagios check execution and use that to populate your RRD data files. There is nothing that comes stock with Nagios to do this. Groundwork OpenSource Monitor which is based on Nagios does provide some functionality to do this. In our environment we use Nagios for monitoring and Cacti for graphing... ie- use the right tool for the job...