When Win Vista ships, you will be able to upgrade to Vista. Some, but not all, situations will allow an "in place" upgrade to be run from your existing Win XP. Others will require a "clean install" where you start from a re-formatted hard drive.
Micsosoft has a web page that describes these scenarios. Here is the link:
http://www.microsoft.com/windowsvista/getready/upgradeinfo.mspx
So, if you are thinking about buying a new Win XP PC before Vista PCs are available, check out the upgrade "coupon" offers carefully, and understand the options. One trick with the upgrade coupons is that you have to upgrade by March 31, 2007. I was thinking of getting a new Win XP PC now and using the upgrade coupon to Vista after several months of Vista experience were in place -- the coupon won't work in that case.