vista coupons clarification

hey everyone. ivee been doing some research about upgrading to vista and i
have a few questions. Ive been lookin for the answers for a little while now
and i just need some clarification on some things. Here goes:

1)To get the coupon to upgrade to vista for a nominal fee, do you HAVE to
buy a pc capable of running it, or just buy the xp software. Ive seen online
stores just selling the software with the coupon. Can i just buy that and
upgrade? or do i have to buy a system with it?

2) Im planning on a building a system. I want to load it with vista home
premium. I want it to be a fresh clean install rather than an upgrade. Is
this possible if i buy media center xp and upgrade to vista using a coupon?
I have read that the upgrade goes from windows media center xp -> vista
premium. Would it be possible for me to buy windows media center xp, load
that on my current computer, and when i get my upgrade to vista home premium,
load that on my fresh newly built computer that has no os on it?

Thanks for the help guys and sorry if im bit redundant!