We currently run a very small GMS server:

Pentium Core 2 Duo
Windows 2000
2 dozen users (out of ~1400)

From the initial response to this service and the increasing requests by many of our users (faculty/staff/students) for this ability, we can see it being wildly popular. What I am trying to find are some decent estimates on what kind of hardware we would need to implement to satisfy the demand of our users for the next serveral years.