There isn't a lot you can do to configure the RMI runtime. You can set the socket timeout value by configuring the server with custom client and server socket factories. See section "5.7. Socket factories and server socket factories" (http://labs.jboss.com/jbossremoting/docs/guide/ch05.html#section-socket-factories) of the Remoting Guide for more information.
However, maybe RMI is not the source of the problem. If your server is running on Windows XP, then you might be suffering from a restriction Windows places on the range of TCP ports it is willing to allocate. By default, Windows XP will allocate ports in the range 1024 through 5000. You can increase this range by setting MaxUserPort in the registry. See http://www.microsoft.com/technet/network/deploy/depovg/tcpip2k.mspx#EBNBG for more details.
There is also a restriction in Windows on the number of open connections, TcpNumConnections, but the default value is 0xFFFFFE in Windows XP.