MS has released a calculator to help you figure out exactly what you will need in terms of licenses and dollars in order to meet your virtualization requirements. They have two calculators, one of which requires Silverlight (guess which one I did). The calculators are VM-technology agnostic (meaning that they are the same whether you are using hyper-v, VMWare, or VirtualBox)
On calculator 2, you can either put in the number of servers you have with the avg. VMs per server, or you can list each server individually, which is more likely to be the best scenario for smaller shops. In the below screencap, I entered in one dual Proc (note that cores are immaterial) physical server running 3 virtual machines.
Virtualization Results Example
It reports that we will need either 3 Standards, 1 Enterprise, or 2 Data center licenses (since Data center is licensed per proc).
The pricing columns report that the cheapest way to do this is with the 3 std licenses, but it you wanted to add a 4th vm, the Enterprise license would be cheaper. If you wanted to expand way beyond that in terms of Vm per physical server and core pair, the data center starts to be more cost-effective.
Both calcs are available here
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