Depends on what you need.
Many organizations have built their work-flow around Windows XP, and a change to Vista would be disruptive for them. Most orgs have played with Vista, and have found that it just doesn't offer enough value to go through the trouble of switching their way of doing things, and have decided to wait for Windows 7, expected in late 2009.
Individuals buying new computers find that their much faster (hardware-wise) new computer running Vista tends to be slower than their much slower old machine running XP, so Vista has gotten a negative rap. Not only that, but Vista is "different", and a lot of people don't like "different". And even for those who do like "different", Vista just doesn't offer that much of a compelling reason to switch, other than a lot of new irritations.
So should you get Vista or XP?
Well, Microsoft has End-of-Lifed (EOL'd) XP, so by getting it, you're getting old, dead technology. It's also sometimes hard to find XP drivers for the nice shiny hardware on your new computer.
On the other hand, I agree with many: Vista is a dog, and me no want. (Many people are saying that Vista is the new "Windows ME". If you know your Windows history, that says it all right there.)
And that's why I run Debian GNU/Linux ( http://www.goodbye-microsoft.com , http://www.debian.org ) and why many others run some other variant of Linux; many prefer the Debian-based Ubuntu ( http://www.ubuntu.com ). If you're having to upgrade from XP to something different, you might as well upgrade to a free (in cost, and Free as in Freedom to do what you want with it, including making copies and giving it away) and arguably in many ways better operating system. You can try a LiveCD (aka, "demo"), which is a complete, functional Linux system that runs from CD, not making any permanent changes to your computer; popular LiveCDs are Knoppix ( http://www.knoppix.com ) and Ubuntu ( http://www.ubuntu.com ).
If you choose to install Linux to your hard drive permanently, you can install one of these variants alongside your current Windows setup, and choose which system you want to use when you boot up (or, you can run Windows within Linux, via an virtual machine such as the free VirtualBox, which is what I do). You may find that with the less-resource-hungry Linux installed, your old computer behaves like a shiny, new, faster computer. (The LiveCDs won't run much faster, because they're running from the relatively glacially slow CD drive; but once installed to your hard drive, you may come to love your computer once again.)
But if you are dead-set on spending money for a new computer, get a Mac. They're swee-e-e-et! They "just work". And they work well. Yes, they have their cons as well as their pros, but all things being equal, I find that Mac OS/X is MUCH better than Windows. And if you have to have Windows, XP and Vista both will run on Mac hardware. But once you try Mac, you won't go back.
(I have a MacBook, which I love. But I love my Debian boxes more.)