command to see how many bytes/sec being sent/received by CentOS server?

I have a webserver hosted at a hosting company, and the webserver's Internet connection is an unmetered 10 Mbps (but hard limited to 10 Mbps, and I've verified that downloading a file from a remote speed test site, it downloads at about 10 Mbps). During high-traffic periods, if the webserver starts to slow down, I want to be able to tell if I'm close to hitting a bottleneck because of the connection speed, or for some other reason. So I'm looking for a command to run (on CentOS 5.8) that can collect data for a few seconds and then tell me how many bytes per second my server is sending and receiving (either total, or over a given interface like eth0 -- my server only has one interface so it doesn't matter). If it's not as simple as one command, then I'll take the next-simplest solution :) but the simpler the better. Thanks!