Microsoft Knowledge Base Article 891128 contains the following summary and introduction:
This article describes the new Remote Installation Services (RIS) changes that are made available on a computer
that is running a 64-bit version of Microsoft Windows Server 2003 Service Pack 1 (SP1)
and how to deploy 64-bit operating system images in an enterprise environment.
Additionally, this article describes the new features and functionality of the Risetup.exe and Riprep.exe tools,
the Client Installation Wizard, and the x8664.osc screen.
The RIS feature simplifies the task of installing an operating system on computers throughout an organization.
It provides a mechanism for computers to connect to a network server during the initial boot process,
while the server controls a local installation of Windows. This can be used to either install the correct configuration
of the operating system on a new computer, or to restore a failed computer to a known operating system configuration.
On a computer that is running Windows Server 2003 SP1, RIS now supports running on a Windows Server 2003 SP1 x64-based server.
Additionally, RIS now supports the deployment of x64 images by using the Remote Installation Service Setup Wizard (Risetup.exe)
and the Remote Installation Preparation Wizard (Riprep.exe)) from a RIS Windows Server 2003 SP1 server of any architecture
(x64, IA-64, and x86).