I thought this link was worth posting. A few days back a new calculator was posted on microsoft.com which makes it easier to assess operating system licensing costs for virtualization use. Note that the calculator only takes account of the host operating system and the operating systems in VMs, not for the applications you run in VMs.
This should hopefully make it clearer when trying to understand whether to buy Windows Server 2003 R2 Standard Edition, or Windows Server 2003 R2 Enterprise Edition with the four-VM usage rights or to use Windows Server R2 Datacenter edition which has unlimited VM usage rights. Also it's worth noting that recently Datacenter changed from being an OEM only offering to being additionally available through volume licensing.