Hope This Helps

Windows Vista Home Basic 32-bit 4GB
Windows Vista Home Basic 64-bit 8GB
Windows Vista Home Premium 32-bit 4GB
Windows Vista Home Premium 64-bit 16GB
Windows Vista Business 32-bit 4GB
Windows Vista Business 64-bit 128+ GB
Windows Vista Ultimate 32-bit 4GB
Windows Vista Ultimate 64-bit 128+ GB
Windows Vista Enterprise 32-bit 4GB
Windows Vista Enterprise 64-bit 128+ GB

Windows XP Home 32-bit 4GB
Windows XP Professional 32-bit 4GB
Windows XP Professional 64-bit 128 GB

Windows 2003 Server Standart Edition 32-bit 4GB
Windows 2003 Server Standart Edition 64-bit 32GB
Windows 2003 Server Enterprise Edition 32-bit 64GB
Windows 2003 Server Enterprise Edition 64-bit 1TB
Windows 2003 Server Datacenter Edition 32-bit 128GB
Windows 2003 Server Datacenter Edition 64-bit 1TB

Windows 2008 Server Web 32-bit 4GB
Windows 2008 Server Web 64-bit 32GB
Windows 2008 Server Standard 32-bit 4GB
Windows 2008 Server Standard 64-bit 32GB
Windows 2008 Server Enterprise 32-bit 64GB
Windows 2008 Server Enterprise 64-bit 2TB
Windows 2008 Server Datacenter 32-bit 64GB
Windows 2008 Server Datacenter 64-bit 2TB

Quelle: Wieviel RAM lässt sich überhaupt unter Windows 32-bit und 64- bit Betriebssystemen adressieren?