I think you're allowed to use the key again on a clean install of the 64bit on the same pc.
I hope this is correct.
But this means I'll need to install an older 64 bit OS (Vista, XP, 7) and then install Windows 8 64 bit after that?
I also ordered a DVD with my download, but I'm assuming that DVD is only the 32 bit version as well. Or is it even possible to have both 32 and 64 on one dvd?
sorry, I'm a newb. I should've known this earlier before buying.
Nope.
See:
Upgrade to Windows 8 - Microsoft Windows
"Can I upgrade from a 32-bit version of Windows to a 64-bit version of Windows 8?
Yes, but you can't do this using Upgrade Assistant. If your PC has a 64-bit capable processor (CPU) but is currently running a 32-bit version of Windows, you can install a 64-bit version of Windows 8 Pro, but you'll need to buy it as a DVD and perform a custom installation. If available in your country or region, you can buy Windows 8 Pro from a participating retail store. You can also buy it online from the Microsoft Store in Austria, Belgium, Canada, France, Germany, Ireland, Italy, Japan, Netherlands, Spain, Switzerland, United Kingdom, and United States.
Please note that the Windows 8 Pro Pack is used to upgrade from Windows 8 to Windows 8 Pro, and is not for cross-architecture installs and does not include any media. If you want to change architectures, purchase Windows 8 Pro.
You won't be able to keep any files, settings, or apps when you upgrade from a 32-bit to a 64-bit version."
If you can come up with the 64 bit installer, you should be good to go.
I don't know whether MS will give both versions in the backups DVDs. (For Windows 7, the official versions came on one DVD each.)
Perhaps you should contact Microsoft. I wouldn't be at all surprised if they accomodated you. (Example: they could email you a download link for the 64 bit installer. It wouldn't cost them anything, except to pay for the time of one of their support people.)