Exchange to DC processor Ratio's

This has become a big topic and I wanted to put this out. 

• If Active Directory is running on the x86 platform (32-bit), the recommended ratio of Active Directory directory server processor cores to Exchange 2010 Mailbox server processor cores is 1:4.
• If Active Directory is running on the x64 platform (64-bit), the recommended ratio of Active Directory directory server processor cores to Exchange 2010 Mailbox server processor cores is 1:8. To achieve the 1:8 ratio, you must have enough memory installed on the directory server to cache the entire Active Directory database in memory. To check the size of your Active Directory database, examine the NTDS.DIT file on a global catalog server. By default, this file is located in %WINDIR%\NTDS.

Please make note of the highlighted section this is important for Virtualized Environments. DC’s must have enough memory to cace the entire NTDS.Dit and maintain the 2 GB of system memory.