If you plan on installing Microsoft's 64-bit version of Windows XP on your Intel Nocona Xeon processors you're out of luck. Apparently the 64-bit version Windows can't run with Nocona extensions. However you won't experience this problem with AMDs Opteron processors. According to an independent hardware reviewer the reason for this is because Microsoft hasn't had time to test its 64-bit OS on any other processors. Right now the Opteron is kicking some serious butt so it makes sense to address it first.
Customers can now purchase workstations using Intel Corp.'s Nocona Xeon processor, with 64-bit extensions to the x86 instruction set, but they can't run the beta 64-bit version of Windows designed for those extensions on the new workstations. The publicly available beta version of Windows XP 64-Bit Edition for 64-Bit Extended Systems can only be installed on systems with Advanced Micro Devices Inc.'s (AMD's) Opteron and Athlon 64 processors, according to a Microsoft spokesman. During the operating system installation process, Windows checks to see whether or not it is being installed on an AMD system. If it is not, the software cannot be installed, the spokesman said.
News source: InfoWorld