Last time I actually brought a processor AMD were loads cheaper for equivalent performance. Assuming that hasn't changed and assuming there's a massive premium for AMD 64 (When isn't there a premium for a new technology?) I'd go standard 32bit AMD.
Of course if it's not loads more expensive I'd go AMD 64.
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He who disagrees with me in private, call him a fool. He who disagrees with me in public, call him an ambulance.
Unless you are really really really PC savy, XP64 is not for you. I run both AMD and Intel systems, and have gotten good service from both. With that said, Intel still has the edge because of their chipsets. You can't go wrong with a P4 in an Intel board.