These boxes are more often called workstations than PCs, but why not
To estimate the performance, just ask the user how they feel about the responsiveness of their applications.
To measure the load, there are commands that display internal counters. The important task is to identify the bottleneck(s), if any, that prevent the applications to run "full speed".
Main candidates for bottlenecks are the CPU, Memory, disks and network.
Standard commands that you can use to see what happen are vmstat, mpstat, iostat, netstat, prstat and some other.
You can also download the free and amazing SE Toolkit (
http://www.sunfreeware.com/setoolkit.html ) that can analyse the load for you and establish a diagnostic, just install it and try "zoom.se".
