Vista is native 64-bit although a "dumbed-down" 32-bit version will be available for those silly enough to want it.
Driver support/program compatability for XP64 has improved dramatically in the past 6 months and will only get better. After all, both AMD and Intel have shifted production completely to 64 bit chips.
BTW, if you've tried the beta XP64 or the free "trial" version, there is no comparison to the full retail/OEM version. The "real" version actually works...the others were just teasers.
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