Do not enter a keycode or activate: I mention this only because should something go wrong during one or more of the installs you can always start over and not worry about activation problems. You have a 30 day grace period to activate so after you are satisfied you have a workable multi-boot computer then you can activate each version of Windows you installed.
Image Backup Software: I mention this only because after you spend all the time and effort to install multiple versions of Windows, image backup software can save you from losing all that effort. Two software applications worth considering are Norton Ghost version 15 and Acronis True Image 2010. My preference is Norton Ghost only because Acronis seems to have more problems in correctly identifying partition drive letters. If you are not ready to buy, both offer trial versions, but for evaluation purposes the Acronis trial version has an advantage in that it lets your create that all important recovery/rescue boot CD.
The install order: Windows XP, Windows 7 Ultimate 32-Bit,Windows 7 Ultimate 64-Bit, Windows 7 Professional 32-Bit, Windows 7 Professional 64-Bit, Windows 7 Home Premium 32-Bit, Windows 7 Home Premium 64-Bit.
After completing each Windows 7 (or Vista) install use EasyBCD to update the boot menu display information. Note that after all Windows 7 installations have been completed the last version and most recent version you installed (Windows 7 Home Premium 64-Bit in this article) will be just below the “Legacy” entry in the boot menu and the first version of Windows 7 you installed will be listed at the bottom of the list. For a seven boot configuration, the seventh entry may not be displayed when the boot menu is displayed and you may need to use the down arrow key to locate it.