If you bought a new windows 7 equipped PC after June 2 2012, you are entitled to purchase an upgrade to Windows 8 PRO for only $14.99 and you can do it now here.
Unlike the older Win 7 upgrade offer which moved you from Vista to a basic home version of windows 7, the new offer moves you to the best Win 8 version available.
Windows Media Center
If you use Windows Media Center, you may be disappointed to learn that it will be a purchasable upgrade in Windows 8 Pro. Windows Media Center is useful when using your PC as a TV connected set top box or if you want Windows to playback DVDs (or you can buy third party software to enable DVD playback).
If using the Windows upgrade assistant, your new OS will be the same bit level as the one already installed. If you have a 32-bit version of Windows 7 then the upgrade will be to the 32-bit version of Windows 8. Same think for 64-bit. If you want to switch, you will have to do a full re-install.
Since most upgrade will be delivered online (via download), it is important to remember that the typical upgrade download will be about 2GB.
During the installation process, one of the options is "Install by creating media". Choose this option to create a DVD or USB Key based installation media (which is a good idea by the way).
Order from the upgrading PC
Make sure you complete the order form from the PC you will be upgrading. Although many have reported that this is not a requirement, it is the method recommended by Microsoft.