The primary motivation for using the x64 ISO was breaking the 4 GB RAM barrier Windows XP Professional x64 Edition | Windows вики
Unlike standard Windows XP, which uses the NT 5.1 kernel, the Windows XP 64-bit ISO is actually based on the Windows Server 2003 windows xp 64-bit iso
(NT 5.2) codebase. This makes it more stable and secure than its 32-bit counterpart but creates a "split" in the Windows XP family: Service Packs: The primary motivation for using the x64 ISO