We are running Domino 6.5 on Windows 2000, with 2.5 GB of memory. Typically, we have about 400 Notes client users and 100 Web client users. And we use Microsoft failover cluster.
Consider looking at Performance Monitor to see where your bottleneck is. It sounds like RAM is good, but check it to be sure. Make certain your CPU isn't maxed out, and check for I/O contention. My guess is that either your CPU(s) are pegged, or your system is not optimized from an I/O perspective (for instance, separation of processes and files among many controller channels).
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