Windows Server 2008

The Windows Server 2008 Standard Edition from Microsoft is an operating system for x64-based and x86-based server computers. The industry-standard for PCs, Windows Server 2008 features the tools necessary to manage a server and associated storage.

Windows Server 2008 Enterprise delivers an enterprise-class platform for deploying business-critical applications. Help improve availability with clustering and hot-add processor capabilities. Help improve security with consolidated identity management features. 

Microsoft Windows Server 2008 32-64 bit Standard with 5 CALs - Download. Mfg Part Number P73-04712

Microsoft Windows Server 2008 Standard provides users with a stable operating system for PCs designed to function as a dedicated server on a network. 

Datacenter Edition is most cost efficient for more than four VMs per processor, because you can run an unlimited number of VMs per processor and license each processor. While Datacenter is more expensive than Enterprise when running four VMs per processor, you have more scalability and support for future growth, so you could, potentially, adopt Datacenter over Enterprise when you consider future requirements. Remember that you have to license all processors in the server.