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http://www.asia.microsoft.com/windowsxp/64bit/Har du brug for følgende, er det værd at overveje - ellers ikke:
Large Memory Support
Windows XP 64–Bit Edition supports up to 16 GB of RAM and 16 TB of virtual memory, enabling applications to run faster when working with large data sets. Applications can preload substantially more data into virtual memory, allowing rapid access by the Intel Itanium processor.   
  Optimized for the Intel Itanium processor family
Windows XP 64–Bit Edition has been optimized specifically for the Intel Itanium processor and benefits from its key features, such as the Explicitly Parallel Instruction Computing (EPIC) design and increased floating–point performance.   
  Multiprocessing
Windows XP 64–Bit Edition is designed to support multiprocessing capabilities for maximum performance and scalability, supporting up to two symmetric Intel Itanium processors.   
  Interoperability
Windows XP 64–Bit Edition provides a rich platform to integrate both 64–bit technical applications and 32–bit business applications using the Windows on Windows 64 (WOW64) x86 emulation layer.   
  Same programming model
Developers with 32–bit skills will be comfortable and quickly productive in the Windows on Itanium environment, finding it virtually identical to the development environment for 32–bit Windows.   
Altså kortfattet - kører du med Itanium processor, får du et OS der er optimeret hertil. Gør du ikke, kan du lige så godt spare dine penge :)  
NanoQ