Great new Windows Server Virtualization License Calculator
Checkout the new Windows Server Virtualization License Calculator up at https://www.microsoft.com/windowsserver2003/howtobuy/licensing/calculator.mspx
This tool can help you identify whether buying Enterprise Edition or Datacenter Edition of Windows Server 2003 is the better approach based on how many VMs you expect to run on each Host machine.
The tool is initially populated with US retail prices but you can override these with you own local pricing to get the right result for your locality.