I have Vista Ultimate. I tried it for a couple of months and frankly, it's nothing special to me. It's decidedly more annoying than XP, in that there are niggly little things like asking for admin permission that occur all too frequently for my taste.
I don't like that the system requirements have skyrocketed since XP. You're likely to see a decrease in performance on Vista relative to the power of the machine, especially if it doesn't have a reasonably powerful GPU.
In my experience it's not as stable as XP yet, the operative word being yet. I think that as time passes, were going to see a decrease in the number of driver-related crashes as developers get to know the system more thoroughly.

However, it is the way of the future for Windows users, and support for XP can only decrease from here. Most software functions similarly in Vista to how it did in XP, but it's rare to see a program that hasn't been affected in some way by the difference in OS.

If I were you, I'd (begrudgingly) accept Vista, and install Linux on another partition.