In enterprise computing, customer requests often need to be distinguished, with different request characteristics and customer?s different service requirements. In this paper, we consider a set of computer resources used by a service provider to host enterprise applications for differentiated customer services subject to a service level agreement. We present an approach for resource optimization in such an environment that minimizes the total cost of computer resources used by a service provider for such an application while satisfying the QoS metric that the response time for executing differentiated service requests is statistically bounded. That is, each ?(r)% of the time the response time is less than a pre-defined value for class r customers. This QoS metric is more realistic than the mean response time typically used in the literature. Numerical results show the applicability of the approach and validate its accuracy.