Windows Server 2003 R2 Datacenter Edition
Big changes with Datacenter Edition.
Windows Server 2003 R2 Datacenter Edition will continue to be the high-end edition for scalability & reliability, and it also became our high-end edition for virtualisation. Datacenter now has the rights to run an unlimited number of virtualised instances of Windows Server.
Datacenter is now available through Volume Licensing, from OEMs on "normal" servers with 2-64 processors, and from OEMs with the Datacenter High Availability Program on high-end servers with 2-64 processors.
Datacenter is the right edition of Windows Server for large-scale virtualisation on any size server. It is more flexible and cost effective that Enterprise Edition if you are going to run more than 4 VMs per processor (read processor socket - we don't mind how many processor cores you have).
More info here.
Dave