More factors are involved if a physical server is a good VM candiadate. You have to look at the role of the server and what applications are running on the system. As such, most systems in my experience are normally good candidates as long as you build your ESX infrastructure with a SAN/NAS backend to store the VMs so space would not be the issue. Just beef up the esx hosts with CPU & Memory to accomodate the various server roles, etc.
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