1. A computer-implemented method of predicting system performance and capacity using a database of performance statistics measured for reusable software modules, comprising: receiving, by a performance simulation modeling tool of a computing system, a set of performance statistics of a test computer, wherein said set of performance statistics is associated with a set of reusable software modules included in a library of reusable software modules, wherein said set of reusable software modules is required for building a target information technology (IT) system, and wherein said set of performance statistics is received from a database of a plurality of performance statistics resulting from a plurality of individual executions on said test computer of each reusable software module of said library; and predicting, by said performance simulation modeling tool, subsequent to said receiving said set of performance statistics and prior to said target IT system being built, a plurality of computing resources required by said target IT system.