enable snmp and use any number of utilities to track and tally your bandwidth usage remotely. MRTG and/or RRD work well, I am sure windows has plenty of commercial GUI's capable of SNMP monitoring routers, its pretty much the standard way of business but can be kinda complicated for home-use.
this kinda tracking is kinda beyond the idea of m0n0, bandwidth tracking & logging requires local disk space and regular writes... plus while only a month's stats may be sufficent for you, I got daily/weekly/monthly/yearly stats polled every 5mins going back nearly a decade, its not the job of my router to do this level of logging.
Another good option is port-mirroring, ethernet hub, or ethernet-tap going to an interface w/ntop listing on it.. its a passive listing device that can give you all kinds of real-time network information without interfering w/throughput trying to process it all. Also makes a good place to stick some IDS monitors to detect hack attempts. However this is even more complicated of a setup for home use, (hint:
http://ossim.net )