Microsoft Active Directory Documentation Team recently published Capacity Planning for Active Directory Domain Services based on Windows Server 2012 on TechNet.
This document has a detailed and comprehensive set of guidelines to help optimize computing resources for DCs, including considerations for virtual environments. It's by Ken Brumfield, a Senior field engineer at Microsoft, based on his extensive experience and research.
Check the full article at Source (broken link at the moment!), use this Wiki
This link is no longer working. Please update the info on time.
No one seems to have a working link of this document...other than the wiki article which also contains broken links
Updated the Wiki Link! Thanks