MCE = BAD BAD BAD
get OEM of plain PRO, but I recommend buying an HD or something with it to be legal. yeas you can buy without, but if you want to stay "legit" buy an HD.
as for upgrade, well, you can get one, but if there ever is a problem you can't just re-install XP, you have to install 2000 then upgrade. I recommend a full version.