XP x64 to Vista x64 Clean Upgrade Scenario

I am building a new computer on which I would like to load Vista Ultimate x64
Edition. I have a valid copy of XP x64 that has never been installed or
activated on another machine, and a valid copy of Vista Ultimate Upgrade
Edition. I have done extensive research on the x64 upgrade process, and
discovered conflicting reports:
Some say you must run the upgrade disk from within XP, others say you can
boot from the disk as long as XP is installed.
Some say it verifies your XP installation online via Windows Genuine
Advantage, other say it merely checks the hard drive to see if XP is
installed at all.
Some say you can format and repartition a drive during the Vista
installation via the Upgrade CD, others say you must do it beforehand in XP.
Can someone who has actually performed the process verify these claims and
let me know what I can expect during setup? There seems to be widespread
confusion and speculation on this topic, and I am sure the x64 community
would appreciate some straightforward clarification. I am trying to load
Vista this way to save some money, and I don’t want to make a $400 mistake.