Este site tem todos os detalhes que você precisa: Atualizar para o 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.
Sugiro que você faça o download e execute o Assistente de atualização. Isso sugeriria o que é certo.
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.