You can install by booting the dvd.
You don't need to do an in place upgrade- therefore you don't need the same bit version to install.
Confusingly, you do need a 64 bit os version to get the 64 download .
Once you have downloaded it - then you can install on a machine running a qualifying 32 bit os.
More oddly, you cab even use the 8wdp, 8wcp, or 8rp.
Upgrade to Windows 8 with only a product key - Microsoft Windows
Make sure you’re on the PC you want to upgrade. This must be a PC that is currently running Windows 7, Windows Vista, Windows XP with Service Pack 3 (SP3), Windows 8 Release Preview, Windows 8 Consumer Preview, or Windows Developer Preview.