What is the maximum amount of RAM the Windows operating system can handle?

Answer: 

That depends on two factors: the amount of memory your computer hardware can handle, and the amount of memory your Microsoft® Windows® operating system (OS) can handle.

First, your computer hardware (motherboard) is designed to hold a maximum amount of RAM. 

Second, the OS maximum is the maximum amount of memory that your particular version of Windows, Linux, or Mac® OS can handle. 

When purchasing your memory upgrade, make sure that you do not exceed the lower of the two maximums (OS and computer maximums.) Too much RAM can lower your system's performance or cause other problems. (In most cases, the system maximum is lower than the OS maximum.)

Windows Vista (32 bit)
Ultimate: 4 GB
Enterprise: 4 GB
Business: 4 GB
Home Premium: 4 GB
Home Basic: 4 GB
Starter: 1 GB

Windows XP (32 bit)
Professional: 4 GB
Home: 4 GB
Starter Edition: 512 MB

Windows Server 2003 (32 bit)
Datacenter SP2: 128 GB
Enterprise SP2: 64 GB
Standard SP1: 4 GB*
Datacenter R2: 128 GB
Enterprise R2: 64 GB
Standard R2: 4 GB*v 
Web Edition: 4 GB*
Small Business Edition: 4 GB* 

* Certain Microsoft server operating systems can support over 4GB of memory via Physical Address Extension (PAE). Please refer to Microsoft knowledgebase article located here for more information.

Windows Server 2008 (32 bit)
Datacenter: 64 GB
Enterprise: 64 GB
Standard: 4 GB
Web Server: 4 GB

Windows Vista (64 bit)
Ultimate: 128 GB
Enterprise: 128 GB
Business: 128 GB
Home Premium: 16 GB
Home Basic: 8 GB

Windows XP (64 bit)
Professional: 128 GB

Windows Server 2003 (64 bit)
Datacenter SP2: 2 TB
Enterprise SP2: 2 TB
Standard SP1: 32 GB
Datacenter R2: 1 TB
Enterprise R2: 1 TB
Standard R2: 32 GB
Small Business Edition: 128 GB

Windows Server 2008 (64 bit)
Datacenter: 2 TB
Enterprise: 2 TB
Standard: 32 GB
Web Server: 32 GB

Here are the OS maximums for some older versions of Microsoft Windows:

Windows 95: 1GB
Windows 98: 1GB
Windows 98SE: 1GB
Windows ME: 1.5GB
Windows NT: 4GB
Windows 2000 Professional: 4GB
Windows 2000 Server: 4GB
Windows 2000 Advanced Server: 8GB with PAE enabled
Windows 2000 Datacenter Server: 32GB with PAE enabled

Here are the maximums for some other platforms:

OS X (including v. 10.4 "Tiger"): 8GB due to current hardware limitations (the current maximum memory capacity of Apple's highest-end system, the Power Mac G5)

OS 9.x: 1.5GB (no single application can utilize more than 1GB)

Red Hat Linux 2.4 kernel: 64GB