Jason's Humble Abode

1. Preparation (pre-requisites)

Before doing anything, BACKUP everything thoroughly either via through stsadm / backup agents. This includes OS, system state, SQL databases and MOSS for rollback purposes :)

Before performing the installation, make sure you have the following :

Microsoft software

1. Windows Server 2003 / 2008 x64
2. x64 OS drivers for SAN, hard disk, backup agent (where applicable)
3. IIS 6.0 / 7.0 (with related x64 service packs or hotfixes)
4. SQL Server 2005 / 2008 x64 (with related x64 service packs or hotfixes)
5. MOSS 2007 x64 (with related x64 service packs or hotfixes)
6. Microsoft Office x86 (compatible with 64-bit) if required.
7. .Net framework 3.0 x64

Custom-built / 3rd party components

1. Check from vendor if 3rd party components are 64-bit compatible. Test thoroughly.
2. Custom built components are compatible with 64-bit if they are built in Visual Studio using the 'Any CPU' option.

Execution 

1. Server OS
- Remove server from domain
- Install OS
- Setup and configure OS
- Install necessary OS patches / hotfixes
- Install IIS, .Net framework 3.0
- Setup necessary users and service accounts
- Join server to domain
- Test network connectivity and remote connection

2. Database server e.g. SQL Server 2005
- Detach all databases from x86 instance
- Backup database files (.mdf & .ldf)
- Install x64 instance of database server
- Attach all detached database to new x64 instance
- Test database queries

3. Application
- For MOSS, ensure that all service packs and hotfixes are identical in ALL WFEs or else you might have trouble joining farm.
- Install MOSS  x64 and related service packs / hotfixes
- Setup and configure MOSS farm
- Install custom / 3rd party components
- Test sites

There's one more thing to note. Backup and have ready all the components / templates / pages / user controls that needs to be deployed to the 12 hive folder e.g. _layouts and each web application BIN folder (e.g. c:\inetpub\wwwroot\wss\VirtualDirectories\80).