This white paper compares performance of the 64-bit Windows Server 2003 operating system and the 32-bit Windows Server 2003 operating system with Service Pack 1 (SP1) for Active Directory operations. It also investigates workloads for which the 64-bit version of the operating system outperforms the 32-bit version.
http://www.microsoft.com/downloads/details.aspx?FamilyID=52e7c3bd-570a-475c-96e0-316dc821e3e7&DisplayLang=en