Sniffing the glue that binds the internet
Ethereal or tcpdump should be fine for you, ettercap is an 'agressive' sniffer which is used for 'monkey in the middle' attacks by re-routing(ARP/MAC poisoning) packets through the sniffer before delivery to the proper client(This is not considered nice if the packets arent yours) Packets are not always broadcast to all channels depending on network design, and packets in another segment of the network(across a router or "smart" switch) will not be sniffable with the standard technique. Read here for router/hub details. There are many other excellent sniffers available such as dsniff and WinDump(tcpdump for M$) here are some links you can use to familiarize yourself with packet sniffing.
http://ethereal.ntop.org/
http://www.networknewz.com/2001/0723.html
http://www.boran.com/security/sniff.html
http://www.robertgraham.com/pubs/sniffing-faq.html
Happy Sniffing,
Maestr0