1. A server system comprising: at least two scaleable tiers of server machines; a server pool including plural spare server machines; means for computing an average response time for the server system to respond to transaction requests at the two scaleable tiers of server machines; and means for increasing a number of server machines processing transactions for each of the two scaleable tiers of server machines by allocating the spare server machines to process a portion of the transactions, wherein the spare server machines are allocated to process a portion of the transactions when the average response time for the server system to respond to the transaction requests is greater than or equal to a specified average response time, wherein said means for computing further comprises a non-iterative queuing model for predicting the average response time for the server system in response to measured arrival rates of transaction requests into each of the two scaleable tiers of server machines, an average service demand at each of the two scaleable tiers of server machines, and a number of servers allocated to each of the two scaleable tiers of server machines.