Here's more questions for the virtualization gurus out there, in
particular those experienced with Red Hat Xen:

What is the optimal hardware for a server hosting multiple guest OSes?
are there certain processors that are preferable? Any to avoid? Is there
a rule of thumb regarding how much physical memory per guest OS? Any
other tips, warnings, advice, cautionary tales, caveats, monitions, etc?

    Thanks much,

                 Eric W., Libraries Systems