SP1 is a free update for all vista users so don't worry about that.
There is the cheap OEM version and the full retail version.
OEM is what you get with a new pc and is locked to the hardware so changing any major hardware component will undoubtedly force a re-activation request and you may have to phone MS to explain your change and plead to allow them to reactivate.
Retail is a lot more lenient and you can change hardware a lot more and it online re-activation normally works straight away.
As you have found out lots of retailers are happy to sell the OEM version on it's own. Means you don't get any MS support though in case of problems so beware of that. Not sure if OEM version will update an XP installation but it is probably better to do a fresh install of Vista anyway to clear out all the chaff.
Boo!