From my experience, going from an Intel chipset to a different Intel chipset, you wouldn't need to reinstall Windows XP. I did an 845 to 875 upgrade, and it booted up just fine. However, if going from say a SiS chipset to an Intel chipset, you'd probably have to do a repair install or just a clean install of XP. I guess XP/2000 looks for the hard drive on the old IDE chipset when it wants to load all its drivers. This causes it to give you a blue screen of death, and won't let you boot completely into Windows.
So, it is generally best to reinstall after changing your motherboard.