I'm in the same boat, and since this is a common question these days I will add some more detail within the same question.
I'm in the process of building a new pc and it will be used for pretty everything, from processing school programming homework, gaming, and other general media around my apt. I have the option of using the latest XP, or the latest Vista Ultimate 64 bit (thanks to my school we get it for $10 can't beat that!). I like the customization that XP currently has and I am satisfied with the current performance as it meets all my needs, however I am not that opposed to Vista as long as I can get the same (if not more) accomplished without much hassle.
Should I just stick with the XP to which I am accustomed, or should I venture into Vista which could/would lead to better performance for my computer? ( I could always have 2 drives and/or dual boot )