Router Maximum Bandwidth Usage

My LAN has three PCs accessing the internet via an ADSL modem-router.

I need to establish my internet maximum bandwidth usage in order to
establish if a "capped" ISP may be more cost-effective for my ADSL
usage. Checking the modem-router configuration info, I am
uncertain which screen provides this information, as the three
possibilities give different data eg:

Parameter Tx Bytes Rx Bytes
ATM Status 187500697 648851334
PPP Status 108313972 544506384
TCP Status 5800499 2277007

Can anyone clarify which parameter I should monitor to provide similar
usage data to that recorded by UK ISPs. I only need to know the amount
of internet traffic, ignoring internal LAN traffic.

Many thanks,

Zill