Good server arhitecture

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C#

Java

server

Hi all, i have created a basic multiplayer online hearts game with a tcp/ip socket connection, server-client. The server is implemented with a listener that can accept connections from users, for each user i have created a thread that handles the receive and the send message from server to the client, and now i want to get some ideas about how much simultaneous users this server can handle and how can i extended it to much more users? using cloud? server OS? what kind of architecture for the server would you suggest so it can handle a lot of simultaneous users? maybe some examples of this type of application: http://www.pokerstars.com/ ; http://garena.com/

If you know some tutorial/book/website about this i will appreciate.

Thanks,
Sas Gabriel