Well, ettercap is usually limited to whatever traffic is picked up by a switch or router. So, if it's a switch with a single LAN, you'll pick up all the traffic in there. However, if it was the main switch for say AT&T, you'd better have a lot of RAM and a huge pipe for it to handle the traffic flow. Sorta think of it like tcpdump but a little more intelligent and more user friendly.

The one thing I'd suggest is checking out the plug-ins included with ettercap. Some of them are quite nifty.