Windows 2008 Physical Memory Limits

Version Limit in 32-bit Windows Limit in 64-bit Windows
Windows Server 2008 R2 Datacenter N/A 2 TB
Windows Server 2008 R2 Enterprise N/A 2 TB
Windows Server 2008 R2 for Itanium-Based Systems N/A 2 TB
Windows HPC Server 2008 R2 N/A 128 GB
Windows Web Server 2008 R2  N/A 32 GB
Windows Server 2008 R2 Standard N/A 32 GB
Windows Server 2008 R2
Foundation
 N/A 8 GB
Windows Server 2008 Datacenter 64 GB 2 TB
Windows Server 2008 Enterprise 64 GB 2 TB
Windows Server 2008 for Itanium-Based Systems N/A 2 TB
Windows HPC Server 2008 N/A 128 GB
Windows Web Server 2008 4 GB 32 GB
Windows Server 2008 Standard 4 GB 32 GB
Windows Small Business Server 2008 4 GB 32 GB


Source:
http://msdn.microsoft.com/en-us/library/aa366778