|I would start with event viewer.|
Also more important is the network cards. The enterprise level cards do a lot of off loading.
If you really do have 300 people with intensive file it might be too little for the task.
Perfmon would be the best way but you might have to set it up completely to find out.
"Best Practices", Event viewer, host file, perfmon, antivirus, anti-spyware, Live CD's, backups, are in my top 10