With Server 2003, Server 2003 R2 and Server 2008 Enterprise editions you basically get 1 baremetal host copy and up to 4 virtual copies. With ESX it's 4 virtual instances.

When more than 2 virtual instances are needed, it is currently is less expensive to invest in enterprise edition. Standard edition is about 2/5 the cost of Enterprise (VL OA).

Virtual instance licensing rights with MS Server Enterprise:

http://www.microsoft.com/windowsserver2003/howtobuy/licensing/r2eelicensingfaq.mspx

http://www.microsoft.com/windowsserver2008/en/us/virtualization-ent.aspx