I think I'm in the same boat as you. I had XP 32 bit and purchased the UPGRADE version of Widows 7 Home Premium. My computer is 64 bit capable. I obviously could not upgrade so I had to do a clean install but it would not let me do a clean install to 64 bit Windows 7, I even tried booting to the 64 bit DVD from BIOS and it would not work.
I did a clean install from the 32 bit Windows 7 Upgrade DVD and it worked fine, so now I'm running Windows 7 32 bit.
Now I am wondering if I can go from Windows 7 32 bit to the 64 bit version doing another clean install using the 64 bit upgrade DVD withoug screwing up the product key, licensing, etc...