Brian,
tcpdump is more a network analysis tool that network performance tool. It really depends on what you want to analyze or where you think the problem is. tcpdump could indeed help you gather information about what's happening network-wise when the user sees the performance problems but won't tell you too much about network performance more than what's going through the wire (and what the computer can listen to). For a good GUI to tcpdump, look at Ethereal.
If you have a router in the network that can output SNMP or a linux machine, then you could gather information from there and plot it with mrtg, it will tell you more about the amount of traffic that you have there.
Just my two cents, hope this helps.
