As VHG says, unless there is some compelling reason (unlikely at the present) to upgrade, then don't! I have a test machine on which I am running Vista, and after running it for 2 weeks, I'm still experiencing problems - up to 10 a day!
What's more, almost every update from MS (and they are frequent - makes you wonder if they should have finished writing it BEFORE they put on sale), causes more problems.
Even Windows ME was more stable than Vista, and that's not a dig at buenchico :-)