Microsoft: Memory Limits for Windows Server 2008 R2

The following table specifies the limits on physical memory for Windows Server 2008 R2. Windows Server 2008 R2 is available only in 64-bit editions.

• Windows Server 2008 R2 Datacenter - 2 TB (Limit on X64)
• Windows Server 2008 R2 Enterprise - 2 TB (Limit on X64)
• Windows Server 2008 R2 for Itanium-Based Systems - 2 TB (Limit on IA64)
• Windows Server 2008 R2 Foundation - 8 GB (Limit on X64)
• Windows Server 2008 R2 Standard - 32 GB (Limit on X64)
• Windows HPC Server 2008 R2 - 128 GB (Limit on X64)
• Windows Web Server 2008 R2 - 32 GB (Limit on X64)

Reference:
Memory Limits for Windows Releases
http://msdn.microsoft.com/en-us/library/windows/desktop/aa366778%28v=vs.85%29.aspx#physical_memory_limits_windows_server_2008_r2