This paper addresses the problem of mapping the requirements of a known media service workload into the corresponding system resource requirements and accurately sizing a media server cluster to handle the workload. In this paper, we propose a new capacity planning framework for evaluating the resources needed for processing a given streaming media workload with specified performance requirements. The performance requirements are specified in a Service Level Agreement (SLA) containing: i) basic capacity requirements that define the percentage of time the configuration is capable of processing the workload without performance degradation while satisfying bounds on system utilization; and ii) performability requirements that define the acceptable degradation of service performance during the remaining, non-compliant time and in case of node failures. Using a set of specially benchmarked media server configurations, the capacity planning tool matches the overall capacity requirements of the media service workload profile with the specified SLAs to identify the number of nodes necessary to support the required service performance.