You are getting different answers because of the different interpretations of the word "upgrade".
The OP is asking if they can purchase an upgrade edition of Windows 7 (64-bit) legally and use it if they have a legal copy of Windows XP. The answer is YES. I did this myself. Had Windows XP (32-bit) previously installed and put in a new drive and installed Windows 7 (64-bit) on it.
When you purchase Windows 7 you get both DVDs for 32-bit and 64-bit. An upgrade edition usually requires having access to the software that qualifies you for that edition - in this case Windows XP. Typically though MS does not require that you have that software installed. In my case, it was on another drive in the computer so there was no prompting, but it may be that Windows 7 will prompt for a qualifying CD/DVD of the prior OS version (as I experienced when upgrading older Windows versions years ago).
The other upgrade that are in the answers here has to do with installing a new operating system on top of an existing one and attempting keep all the existing applications installed. This would entail migrating/merging any registry entries, replacing and/or removing old files. This is not supported for 32-bit to 64-bit upgrade.
If you attempt to install either the upgrade (reduced price) 64-bit or full price 64-bit on top of Windows XP 32-bit it will not be able to do the in-place upgrade. But then, only Vista 64-bit to Windows 7 64-bit (or 32-bit to 32-bit) upgrade is supported (Windows XP always requires a clean install, but you still can purchase and install the reduced price upgrade edition).