Windows Vista Hardware Assessment
Don't know if anyone's used this but it's a nice helpful little utility.
Install on a PC, run it, enter a couple of settings (Domain, Admin Password etc), leave it half an hour or so.
It will scour AD and the computers generate a Word document and an Excel spreadsheet.
The Word document will give you some nice little pie charts and descriptions listing how many computers need upgrading to run either Vista Basic or Premium and what they need to do it.
The spreadsheet will give detailed information on what computers are running what version of Windows and the SP level, which ones need what hardware etc etc.
Definately worth checking it out: