This website has all the details you need: Upgrade to Windows 8
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, but you'll need to buy Windows 8 as a DVD.
You also won't be able to keep any files, settings, or apps when you
upgrade from a 32-bit to a 64-bit version.
I suggest you download and run the Upgrade Assistant. It would suggest what's right.
It provides the option to buy, download, and install Windows 8. If
your PC is ready to go, Upgrade Assistant provides a recommendation on
which edition to buy, and walks you through the steps to upgrade.