If you are still clueless about the question, you need not worry after the introduction of Windows Anytime Upgrade. With the customer upgrade system for Vista in place, switching between Vista editions is just a change of activation key plus maybe a small download.
As of today, there will be five versions of Vista - Starter, Home Basic, Home Premium, Business and Ultimate. If you look at the differences between the editions, it is obvious that most of the features in the higher editions aren’t needed in everyday computing. Microsoft is noble to let us start off with a basic version and then gradually upgrade as needed.
How does it work? One way it will work is the DVD or the PC that you buy comes with a complete set of Vista OS files. Features from higher editions will be automatically disabled by the OS upon checking the activation key. This approach exposes Vista to cracker’s attacks.
The other way, most probably Microsoft preferred way, is the upgrade software only get authorized by Microsoft server to download additional OS files unique to the edition you purchase.