Well, MS is no longer officially selling XP to the general public, so your choice is either Vista or to hoping that MS actually comes through with a new operating system in another year, which is a questionable wager.
If you are going to be installing it on new hardware, you should have no problems. Most problems result from Vista+Older hardware/software.
I use Vista 64 ultimate on a dual boot with XP, and have compeltely abandoned using XP. Its got some definitely annoying aspects. Most of these aspects can be tweaked or disabled, after which it’s not that different from using XP. Its always going to perform a bit slower at things like old fashioned 2d menu draws and some games, if that is an issue.
On the plus side:
It will work better with zbrush for you, if you have large amounts of Ram. The max amount of Ram XP can use is 2gb per application. Vista 32 can use 3gb, and vista 64 bit (which, if you are upgrading, and going to go through Vista issues anyways, you should think about getting) can use the full 4gb, if you’ve got it.
What does this mean? Measurably higher per mesh polygon counts in ZB, which makes a big difference in quality.
I recommend upgrading to 64 bit Vista with 4gb of ram, and if MS really does come through with a better option in another year or so, you can always upgrade your OS at that time, which will be a very small cost in relation to the big hardsware investment you’re making now.