Routing internet through single PC to monitor bandwidth usage

I'm trying to find a way to monitor all my flatmates internet next year. I've got a semi-decent computer lying around collecting dust that I figure I could use with two network cards to monitor the internet usage of the entire flat. The setup would be DSL Modem -> Monitoring PC -> Switch. Does anyone know of any free programs that'll do anything like this? Short of buying a new router that can run DD-WRT or Tomato, I don't know what to do. Searching only brings up expensive commercial software for monitoring hundreds of computers :c

Your router doesn't measure usage?

Nope :\

Most routers don't. You can upgrade to custom (unofficial) firmware on some of them to allow for bandwidth logging, but that could brick your router if it hasn't been tested. My D-Link DIR-615 is a version 1 so I can't use custom firmware to check bandwidth usage or I will definitely brick my router.

http://www.smoothwall.org/

It should turn your pc into a router, although you can't use it for anything else then.
Its quite easy to set up and runs linux, it has a browser based setup so its easy to maintain from a different pc.
+ It might be possible to add other stuff to this pc as its running linux so you might be able to turn it onto a webserver and do some other things with it as well.

note: if your pc has some issues then the internet in your flat will be down... I think its better to get a 100 dollar router like an asus n-16 and put tomato firmware on it.

Thanks! A mate is moving in with us next year because his flat is splitting up, and it turns out they had a pretty flash router that everyone else forgot about. So I might not need this after all, but if the router can't run DD-WRT or Tomato well, I'll definitely give this a go.

Or if you have a spare PC... x86 OpenWRT...

The added benefit is having a kickass router.

I've managed to score a linksys E3000 router which seems pretty beefy. I'm tempted to just stick DD-WRT on but the process is pretty daunting and I really don't want to brick this router since it doesn't belong to me. If I can get this router managing traffic i'll use that PC as a media server which means I won't have to leave my PC on all the time :neckbeard:

e: I might see if I can get ntop running on that media server once everything is setup. It's going to be a bit tricky though, with 5 PCs, a media server and 2 consoles. Hopefully it's possible to still monitor usage by MAC address even when it's had to go through a switch before hitting the router running DD-WRT.