Options To Monitor Internet Traffic?

ISP has recently announced that they are going to start charging $1 for every GB that their customers go over their monthly limit. i have a 100GB/mo package and i figure that's sufficient for my habits which consist of approx 40GB of torrents, a bit of skyping, bit of browsing, etc. my ISP makes usage graphs available online, when i checked earlier last month to confirm i'm staying under my 100GB limit the graphs said over the past 12 months the highest i topped was 90GB, usually in the 70's. pretty much what i expected. but when i check their usage graphs again recently it says i've been using over 400GB for the past 3 months straight (as far back as the new graphs go). i call BS and find it very suspicious that my internet usage magically quadruples the same time my ISP wants to start charging for overusage, especially retro-active.

anyhoo, i want to somehow monitor my internet traffic usage independently but i'm not sure the best way to go about it.my home network is the typical modem -> router -> LAN.my router is a new netgear wndr3700, it has a traffic meter built in, but considering it's only forms of notification is cutting the internet off altogether or changing a green blinking LED to and orange blinking LED when the defined limit is reached, it's kinda useless. i just got it a couple months ago too, so buying a new router with better traffic monitoring isn't an option.i could put a net meter on each and every computer i have, then sum up their usage stats, but that would be a pain in the ass and wouldn't take my PS3 or WDTV media player into account, as little as they're used online.my prefered solution is to use an old laptop i hardly touch anymore and set it up as a gateway and share the internet connection or make it a proxy server. that way i could set up a net meter on just the one NIC on that to track my internet usage. modem -> laptop -> router -> LAN. the problem i have hear is forwarding ports from one NIC to the other so they can then get ditributed by the router, never tried this before, not sure how to go about it. right now it has XP home, but i can put 7 on it if that's better to work with.