ImageX is a disk imaging tool, sort of like Ghost. You don't literally need it in your setup, but it could be useful.
Suppose you use Virtual PC to make two clones of your SysPrepped VHD, then booted up both copies and ran through the OOBE wizard. You would have two very similar VMs, but with unique GUIDs, etc. In this scenario, you didn't need ImageX.
What ImageX does give you, however, is the ability to edit a SysPrepped image without a lot of hassle. You can use a tool called Windows SIM to insert an answer file into an ImageX image, which allows you to blow past the OOBE wizard. You can use a tool called DISM to do things like add device drivers or enable/disable Windows features in an ImageX image.
ImageX and the rest of these tools are part of the Windows AIK, which is a ginormous download that must be burned to a DVD.
The point of the Windows AIK is to facilitate deployment of Windows clients in a large organization, so it is not geared toward your situation.