I have XP on my desktop Friend but Vista on my new laptop. I expected all sorts of hassles with Vista going on what I'd read, but to be honest I've had no concerns at all. The change over was quite smooth. I believe that most of the initial problems with Vista have now been ironed out now anyway.

I would go Vista on a new computer.

Do you have XP installation disks for the computer you're using at the moment? If so, you could always use them to have XP installed later should you not like Vista for some reason. Besides...... as Southie said, why buy an old product that will soon have no updates or service available?