I don't know anything about Vista except it being a bit of a memory hog, but if its anything like Microsoft's normal new OS's, they can be a pain in the backside whilst they sort out all the bugs.
If you got XP loaded up and then decided you wanted Vista later on. then you will be up for the cost of an upgrade disc, a way around this if you want want to keep Vista and use it now, then if you have a spare Win 2000 or an XP OS disc, which most will have when upgrading, just get them to run a dual boot on it with Vista as being the 2nd one loaded, so you can still run any programs that aren't compatible with Vista on Win2000 or XP whilst Microsoft fixes up all its bugs on Vista.
This is the way i have my PC setup, a choice of win 2000 pro or XP home edit'.
good luck
Brian