Processor:
x64 architecture-based computer with Intel processor that supports Intel 64 architecture (formerly known as Intel EM64T)
AMD processor that supports the AMD64 platform
Intel Itanium IA64 processors not supported
Memory
Minimum: 2 gigabytes (GB) of RAM (Recommended: 2 GB of RAM per server plus 5 megabytes (MB) of RAM per mailbox)
Disk space
At least 1.2 GB on the drive on which you install Exchange
File format
Disk partitions formatted as NTFS file
DVD-ROM drive, local or network accessible
Screen resolution
800 x 600 pixels or higher
Operating System:
Windows Server 2003 Standard x64 Edition operating system with SP2
Windows Server 2003 Standard x64 Edition with SP2, with Multilingual User Interface Pack (MUI)
Windows Server 2003 Enterprise x64 Edition operating system with SP2
Windows Server 2003 Enterprise x64 Edition with SP2, with MUI
Windows Server 2003 Datacenter x64 Edition operating system with SP2
Windows Server 2003 Enterprise x64 Edition with SP2, with MUI
Windows Server 2003 R2 Standard x64 Edition operating system with SP2
Windows Server 2003 R2 Standard x64 Edition with SP2, with MUI
Windows Server 2003 R2 Enterprise x64 Edition operating system with SP2
Windows Server 2003 R2 with SP2, Enterprise x64 Edition with SP2, with MUI
Windows Server 2003 R2 Datacenter x64 Edition operating system with SP2
Windows Server 2003 R2 with SP2, Datacenter x64 Edition with SP2, with MUI
64-bit edition of the Windows Server 2008 Standard operating system
64-bit edition of the Windows Server 2008 Enterprise operating system
64-bit edition of the Windows Server 2008 Datacenter operating system
Microsoft .NET Framework
Windows Server 2003-based computers require both .NET Framework Version 3.0 and .NET Framework Version 2.0.
Windows Server 2008-based computers only require .NET Framework Version 3.0.
Source: http://technet.microsoft.com/en-us/library/aa996719.aspx