Looking to upgrade today? Microsoft Store has the Microsoft Windows 8 Professional 64-bit Software (Upgrade Version) for a low $39.99 Digital Download. Tax in Many States.
Note that this is for the UPGRADE version, not the FULL version. What this means is that you only need to have a working, licensed copy of XP, Vista, or Windows 7 to use this software. If you do have a working copy, however, you can still choose to do a complete, clean re-install if you don't want to save your data or settings.