| Are you planning on running the 64bit system in 64bit mode, or in 32bit mode? If you want to be fully 64bit, then you need the x86-64 version of Windows XP Pro too. Personally, I'd suggest against going to the 64bit OS just now, as there's still some issues with compatibility with some 32bit apps.
The 64bit systems will run quite happily in 32bit mode, although you won't be taking full advantage of them - think of it as future upgradability!
What kind of applications do you intend to run on the system?
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