File server capacity planning and performance troubleshooting are critical aspects of high-level network administration. File server capacity planning tools can be valuable in choosing new hardware for purchase, identifying the capacity of existing hardware, locating existing bottlenecks, and planning for resource expansion in advance of resource exhaustion.
The throughput capacity of a file server can be expressed either as the maximum number of operations per second or a maximum number of users supported by the configuration. These values are influenced by several factors, some of which include processor speed, available memory, disk speed, network throughput and latency, and the speed with which SMB requests are processed.  Source: http://www.microsoft.com/en-us/download/details.aspx?id=27284 Great article: http://windowsmasher.wordpress.com/2011/11/16/file-server-capacity-tool