Windows Server 2008 R2 Datacenter Edition allows you to run unlimited virtual instances. Does that mean I should always use it?

Not necessarily. Windows Server 2008 R2 Standard and Enterprise Editions are both licensed on a per-instance basis, which means that a powerful computer and a less-powerful computer can be licensed for the same cost. Windows Server 2008 R2 Datacenter Edition is licensed on a per-physical processor basis. This change means that servers with greater numbers of physical processors require a more expensive license.
Recall that Windows Server 2008 R2 Enterprise Edition adds four virtual licenses to every physical license, while Datacenter Edition adds unlimited licenses. Recognize also that more concurrently running virtual machines will require more processors to ensure good performance. Thus, a tradeoff between concurrently-running VMs and physical processors is necessary in order to find the best answer. Microsoft provides two different Windows Server virtualization calculators that can assist in determining which license makes the best sense.

source (techtarget.com)

Visit more in http://www.systemadmintools.in/