According to the Microsoft Store, when you purchase the retail upgrade you will receive both the 64 bit and 32 bit disks (see
http://store.microsoft.com/microsoft/Windows-7-Home-Premium-Upgrade/product/B0F9E641). There is absolutely no reason to waste your money buying the full version, as long as you have a valid (and activated) OS. Since XP is a qualifying OS and is presumably activated, that is pretty much all that matters (other than the fact that you will need to do a clean install, but you already recognize that anyway)....
Stephen