I'm not sure on the keys. Never been an issue for us - we use 7 Enterprise on Software Assurance so everything gets upgraded.
Going slightly off topic, ghost imaging isn't recommended nowadays. Microsoft created imaging tools for Windows 7, in fact the DVD installer uses the simplest version. We binned Ghost and the disk cloning machine we used for XP and now network build everything via SCCM. For smaller environments like yours the free MDT tools would provide most of the functionality.
To vastly simplify it, in essence you can build a reference machine with all the apps installed, MDT can then strip the hardware information out and capture a sysprepped image file. You can then deploy that image out in a task sequence, installing the correct drivers on demand for the PC, adding to the domain and running scripts if neccessary.