If I'm performing a an upgrade to Windows 8, do I have to have the previous version installed, or is just having the key enough?

You can install the upgrade without previous OS installed:

You just need to make a Windows 8 Pro upgrade install media and then boot from this disc and install the system (classic windows install procedure). If you get an error when activating (0xc004f061) then call MS support and they will help you to activate the system (you will need previous OS product key and the new Windows 8 PK).

How to create Windows 8 Pro upgrade install media? Download the Upgrade Assistant from http://windows.microsoft.com/en-US/windows-8/upgrade-product-key-only and then run it on a Windows 7 or Windows Vista computer (Windows XP won't work). In the wizard select "install by creating media". It will download the files and then create and ISO file. Burn this file on DVD and you'r done.

It says here that you must be running Windows XP SP3, Windows Vista, or Windows 7 to upgrade to Windows 8.