Because of the move to 64-bit architecture, no in-place upgrades to Exchange Server 2007 are possible. Here's a quick look at some of the minimum requirements your organization must meet before you can migrate to this version.
Step 1: Choose a Processor
Exchange 2007 requires 64-bit processors: either AMD64 or Intel Extended Memory 64 Technology (EM64T). Microsoft has specific recommendations on the number of processor cores necessary according to the role or roles the server will host.
Step 2: Choose an OS
Exchange 2007 has only two options for OS: Windows Server 2003 x64 or Windows 2003 R2 x64. Exchange 2007 management tools can run on some 32-bit Windows OSs. Longhorn Server support will be added with the release of Exchange 2007 SP1.
Step 3: Design Your Network
At a minimum, your network must contain an Active Directory (AD) forest and a DNS server, and your Exchange server must be a member of an AD domain with a domain functional level of Windows 2000 Server native or higher.
Step 4: Determine How Much Memory to Install
Memory calculations are based on the role or roles a server will perform combined with the number of processor cores on the server; for most server roles, you'll need 1GB per processor core, with a 2GB minimum. Mailbox servers generally require more memory, which is calculated according to the number of storage groups (SGs) the server will host and your email users' level of activity.
Step 5: Assess Disk Resources
You'll need to make sure your hard disks have enough space and that volumes containing Exchange databases or transaction logs are formatted as NTFS. Using local continuous replication (LCR) will require additional volumes for the replicated data.