There's several types of software out there that will allow you to monitor usage... in genera. But your standard router doesn't have the more sophisticated management functions that a server might. Here are some options...
IPCop Track4Win PingPlotter
There's something called a "
clark connect box" that can serve that function. How that works exactly, I don't know but the link should help you.
And, if all else fails... buy a new router!

There's a
Belkin router that's supposed to have display that'll give you that info. I don't know which one it is but you can browse their site or contact them.
Lyte