If you are happy with the maths the DG834 Router Statistics page, gives the average Tx, Rx rate for that session.
This would include all access from all computer devices.
http://www.thinkbroadband.com/tbbmeter
tbbmeter can track usage for all the PC's and aggregate them online, if you are XP, of 32 bit Vista/Windows 7 based then you can get the beta (upon request) which can ignore LAN traffic and only count upload traffic. The beta also has an IP tracking page, so you can assess what site it using all the traffic.
If on 64-bit it will work, but the installer for the latest beta has given people a few issues I need to resolve.
If you have an old network hub, rather than a switch there is a standalone app that can actually monitor all the traffic that goes via a hub (my old DG834v1) actually is a hub, rather than a switch, and this lets me assess the traffic usage from my PS3.
http://forums.thinkbroadband.com/tbbmeter/t/3747495-...
This 'snoop' mode is disabled by default on the WAN/LAN beta, but tick Monitor all MAC. If the network is a busy one, best to run that mode on a relatively fast computer.
Andrew