Monitoring bandwidth usage

So we've recently started sharing out wireless internet with the folks upstairs. I've noticed that our internet is getting sloooooow but sometimes that just happens (our ISP blows goats). What would you guys recommend is the best way for me to get an idea how much of our bandwidth they are using?

Stuff I know

1) No unauthorized machines are accessing the router (AFAIK)
2) Router is a Linksys WRT54GS with just the regular firmware
3) Logging is enabled but it only tells me IPs/ports, not amount of data.
4) I have both a wired and a wireless connection to the router (wired is my Windows desktop, wireless is my MacBook Pro).

If the native firmware doesn't provide per IP traffic logging or SNMP monitoring, I don't know how you'd do it without installing special firmware like DD-WRT. Their traffic goes to the router and out to the net, it doesn't pass anywhere else so you can't easily capture and measure it.

Another reason I never use routers like this. I use access points for wifi and a Linux machine as a router. I can very easily isolate and monitor any IP or subnet that travels through my network, whether internal or external. SOHO routers just aren't built for that sort of control (because most SOHOs don't need it). But when you do need it, you have a problem. The good news is DD-WRT, from what I've seen, is quite powerful and capable of doing such things. I've installed it on a couple of Buffalo wifi routers I was experimenting with for another project awhile back, and while I never implemented them, I was impressed at the functionality.

Yeah, I'm slightly paranoid of bricking this router so I was thinking of trying DD-WRT on one of my older Linksys routers to see if that works, at least as a trial run.

DD-WRT is great, and will do this, and it's kinda hard to brick the router. just do it.

it's pretty easy to go DD-WRT if you have the right guides )

Cool, I've upgraded to DD-WRT on my old Linksys router (it's a WRT55AG). The wireless didn't use to work on it so I wasn't using it anyhow, so it didn't matter if it bricked. But I got DD-WRT to install and now the wireless is working on it again - plus I can watch the bandwidth. I'm going to delve more into what I can do with this firmware to try to see how much bandwidth they are using. Thanks for the support guys.