Unless you have some compelling reason for wanting or needing Vista, I would suggest staying with XP. On every computer I have seen, Vista is noticably slower than XP. I've even heard people who bought brand new, powerful systems with Vista say that their old computer was faster. This is especially true for laptops.

I have not used Vista for any length of time on any of my own computers, but I can speak for computers I've set up or worked on for others.