Actually neither of those statements is entirely true.
You can upgrade without even being in possession of a copy of XP. Just install it clean and when it asks for your CD Key, skip that step. It'll install the 30-day trial. Once that is installed, you can do an upgrade install with your CD key and it will count your trial version as a previous version of windows to upgrade from. Takes a little while, but it's a way to get a clean install from the upgrade disk.
Also, regarding invalidating your XP key, obviously with the above method this point is moot, but if you actually DO an upgrade, there has been NO evidence that your XP key is in any way invalidated. It says in the EULA that it will be, but nobody has been able to show that this actually happens. (Though I'd probably still go the above route just to be sure.)