Pentium D is a slightly different architecture based on the Pentium design, that virtualizes a second core. It's proto-dual core. (I know this stuff from Wiki, yo.)
MicroXP is an option in the TinyXP image. It goes slim, tiny, micro, then I think beast. I've been using TinyXP, mostly tiny, since '09, which most of my MAME cabs have. My main PC, theatre, and music rigs use 7 (32, 64, 64 bit).
Unless you're going to get a dual core/Core 2 Duo, and do 7x64, don't bother doing anything.
Scifi frauds. SF illuminates.
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