I've gotten several questions lately regarding virtualization and capacity planning with regard to ArcGIS Server. Below are my answers to some of these questions.
Q: What is ESRI’s official supportability of virtualization of ArcGIS Server?
A: ArcGIS Server is supported in a virtualized system. See: http://www.esri.com/software/arcgis/arcgisserver/common-questions.html. Go down to Standards | Interoperability | IT | Security. The third question down deals with virtualization. Basically, virtual systems are treated the same as physical systems.
Q: What versions of Microsoft Windows are supportable deployments? 32 or 64bit support?
A: The same as support for physical systems. http://wikis.esri.com/wiki/display/ag93bsr/Web+ADF+for+the+Microsoft+.NET+Framework.
Q: What kind of resources (RAM / CPU / Disk) do I need to allocate to ArcGIS Server?
A: It all depends on the services that you are using, the applications that consume those services (desktop, web/server, web/browser), and the number of users that will be interacting with the system. The book Building a GIS discusses how to use the Capacity Planning Tool that you can use to estimate capacity. You can find info on that here: http://esripress.esri.com/display/index.cfm?fuseaction=display&websiteID=141&moduleID=27. To get a more exact picture of performance and scalability you should test an application in a configuration to see how many users it will support. There is information on how to do this here: http://resources.esri.com/arcgisserver/adf/dotnet/index.cfm?fa=mediaGalleryDetails&mediaID=6D73B2DB-1422-2418-344143680A5154BA
Q: Are there any special considerations for virtual systems?
A: The one performance issue that usually plagues VM’s is disk IO. ArcGIS Server tends to do a lot of disk IO. Geoprocessing services do the most disk IO but even a simple dynamic map request involves a write to disk. So improving the performance and scalability of your disk IO will have a significant impact on system performance. For this reason, I prefer to put my ArcGIS Server, server directories on a SAN. I’m not talking about a virtual disk on a SAN, I mean the SAN is attached directly to the VM. This way, you get the fast disk IO from your SAN without the dealing with the disk IO of the virtual machine. This also makes it easier to allocate space for the machine and it makes it easy to share the same server directories between VMs.