As mobile agents can dynamically adapt to resource capacities they suit heterogeneous environments. That is, agents are not bound to a single host and can migrate to an outperforming one. Consequently, agent technology can balance the resources of a heterogeneous system. However, to balance resources efficiently agents must take resource load into account. Thus, to support the agent migration strategy the application of server selection systems has been proposed recently. Server selection systems keep track of the load of network and host resources and hence predict the performance of different migration strategies. Yet, server selection comes at a cost and therefore agents must take care when applying it. This paper presents a server selection system which meets the requirements of mobile agents. Moreover, in contrast to any other server selection system known from literature the new one addresses the problem of rational server selection.