Well hopefully I can answer some of your questions...
To my knowledge all of the editions of Vista are contained on one DVD.
In addition, you are free to try any of them for 30 days if you DON'T enter the CDKey (minus the efforts of re-formatting).
However, the CDKey is only valid for the version you have pre-purchased and I am not sure if Microsoft would discount you if decide you prefer the more expensive versions instead. :roll:
The best thing you can do is phone Microsoft and ask about the exact differences of each and what their volume licensing entails.
It really depends on how much of a power user you are, as you go down in price you lose those extra features like remoting, cpu/memory caps, aero, encryption, backup/restore, media center... etc. 8O
I'm a software developer, by nature I dislike limitations, hence why I am currently running Vista Ultimate RTM.
btw What's wrong with your XP Pro? If its not broken don't fix it...
been there, done that. :x
If I were you, I would wait and see how the 64-bit Vista driver support plays out with HD content and Directx10 support if that's your goal.