> with 8 GB you should add another processor there

I also think, to make use of those 8 GB, you need at least one another processor, even a quad machine is not an exaggeration IMO.

> "Concurrent users" is a difficult term. I usually measure the amount of httpd threads.

Intuitively, "concurrent users" are the number of users waiting for their requests to be finished at any given time. I believe this number agrees with the httpd _processes_ rather than _threads_, depending on the memory model you use.

Coming back to the original idea of having a central server: It will definitely be simpler to maintain one server rather than one server per school. What I don't like about the idea is the fact that all the schools are in one timezone, in one adminstrative entity. It is likely that all will have end of the year exams in the same week, during the same school hours. So the machine will be subjected to very heavy peak loads, provided that the network can concentrate all that traffice to one single network interface!