Ill clear up a little confusion. Windows XP's install is based on a point system. GPU, MOBO, get somewhere around 3 points. Hard drives are 2, ram is 1 (if I remember correctly). After so many point changes (5-7) then it requires you to reinstall windows. However, with things like a mobo change, its recommended you reinstall for compatibility sake.
Vista originally worked on a mobo system. You could change out the GPU, ram, etc as many times are you wanted, but the system was locked to that one and only motherboard (so if it died, you were f....). Anyway, with the latest Vista however, they are using a slightly modified system that I'm afraid I dont know how works.
Anyway, its already been answered, but RunAway, no, you cant sell your copy. I wish I had gotten 64 vista as well, but 7 comes out in november. Get 64 when 7 comes out.

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Edit: Oh yes, for XP and Vista, if you just give microsoft a ring at reinstall time and tell them you destroyed the old computer, doing a routine reinstall, or something similar, they'll give you a new key. Ive done this every year for the past couple of years

! Its legal because its only being installed on one pc

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