These are not "worthless opinions" he is stating a fact. If you want to take full advantage of the 64-bit processing, you will need to buy the 64-bit version of Windows XP. Without the 64 bit XP, you will be processing in the 32-bit mode.
With either OS, your dual core processor should work. But the only way to take full advantage of the 64-bit processing is to install a 64-bit OS.
The link you provided states that XP Profession supports up to two sepparate processors. The x2 is a dual core processor, which is only one processor with two processing cores.
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