NetCrunch allows you to set the thresholds for the SNMP counters. You can monitor the counters related to errors in packets on each device or on a switch to find the segments with the most number of error packets.
Regarding the "Top Network Talkers" you can monitor the appropriate counters on most important devices such as Bytes Transmitted and Received.
If you don't want to use the SNMP protocol you can try to read the system- specific counters such as Netware and Microsoft.
