Among many overlay multicast algorithms, there is one that uses powerful and dedicated servers as multicast routers in the application level. The multicast tree is constructed on the servers, which is referred to as server-based overlay multicast (SOM). Clients who wish to join just access one of the servers. The SOM is adequate for real time multimedia applications, such as video conferencing. In this paper, we want to find the answer to the following question: "What is the best form of network topology that has minimum cost and maintains a minimum quality of service (QoS) of multimedia application?" To answer this question, we made analytical models of SOM with three different topologies-linear, star and hybrid types that combine the linear and star topologies-and compared their performances. We also tried to answer the maximum number of clients that can be served by a group of servers for a given QoS such as tolerable delay. We validated the analysis with an extensive simulation.