[H]ard|OCP have been hard at work comparing various games on an Athlon 64 with both 32bit and 64bit versions of Windows.
What we found isn’t too shocking really, but rather reassuring. In all the 32-bit games tested, we saw overall static performance using Windows XP Pro x64 Edition. The only game in our lineup that had a specific Win64 AMD64 instruction path, The Chronicles of Riddick, actually performed worse in 64-bit than it did in 32-bit Windows.
Oh dear Riddick! Still, at least there doesn't seem to be any harm to performance upgrading to WinXP x64 (mostly, anyway.)

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