If it works like it did when you upgrade XP 32 bit to Vista 64, it would work fine if XP is installed. It however would NOT install if you tried without XP installed first.
It does not work like older versions of Windows where it simply asked you to insert a disk to verify you have the previous version....... the previous version has to be installed before the upgrade will work. Kind of a pain in the ass, but that is the way it is since Vista came about.
It will do a clean install though, since you cannot actually upgrade a 32 bit OS to a 64 bit OS, the 2 OS's are not compatible at all.
The install will save your Windows install to a folder called Windowsold, and will do a completely fresh install.
Although the data is all supposed to be saved here, just to be safe, save it somewhere else as well. If you have data scattered around the drive in odd folders, the install process may not pick it all up, and it may not get moved into the Windowsold folder. If it does not, it will be deleted.
Again, all this is based on how a Vista upgrade worked for XP. I don't have 7, but it would surprise me if it worked any differently.