You can track the "Bytes Received/sec" and "Bytes Sent/sec" counters of the "Network Interface" object(s) that correspond to the NIC(s) that you want to track. That's going to give you pretty graphs on-screen. If you want long-term data for trending purposes or further analysis track the same counters but use a "Data Collector Set" to write the data to disk process it from there.
As a quick-and-dirty trick I use the "Network" tab of "Task Manager" to do the same thing, too. When I'm watching a box that's "locked up" or "frozen" (as users often report) I find that adding the "Bytes Sent/Interval" and "Bytes Received/Interval" columns using the "Select Columns..." functionality can be very handy. Often this lets me see if a machine is moving around a lot of data when, ostensibly, "frozen". Obviously, I also get quick access to CPU and memory details, too. Yay for Task Manager!