Although no one has responded directly to my original question about
scalability, there have been a number of answers posted to other
question that pertain to my question, and articles on the web that
indirectly touch on it. Can someone confirm or correct what I think I
am learning?

The "classic" model defines the heavy weight vertically scalable
configuration for firebird. It is most appropriate for scaling for
environments that are architecturally similar to the original VAX
cluster, such as some recent developments in the Opteron world. It is
also more appropriate where: higher performance is required, the number
of concurrent connections is on the same order as the number of CPU's,
and there is lots of available memory to run the heavyweight connection
processes.

The "superserver" model defines a light weight scaling configuration
that is most appropriate for environments that are architecturally more
similar to the Intel hyperthreading model, or at least dis-similar to
the VAX cluster architecture. Superserver will allow a well built
application to function against a lighter weight server instance, with
only a limited performance penalty. Superserver is less robust, and
demands that application code pay more attention to thread safety since
connections and connection resources tend to be pooled.

Based on these principles, recent notes in the mailing list that a low
end server class machine can safely handle 50 users with 2 CPU's and 2
GB of memory, and empirical knowledge of some relative platform scales,
I have some preliminary "guesstimates" for scaling the classic model:

user count CPU count/arch RAM size OS Storage
50 users 2/Intel 2GB RAM linux, Windows RAID
100 users 2/IBM PPC4 4 GB RAM linux, AIX RAID
500 users 6/IBM PPC4 6 GB RAM linux, AIX SAN
1000 users 16/IBM PPC4 8 GB RAM linux, AIX SAN
10000 users ?/ES9000*? 128 GB RAM linux, AIX SAN

* Note that scaling to the ES/9000 may not be at all straightforward