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Don't use it if your on Vista as it will mess your system up.
ATI to ATI or Nvidia to Nvidia, NO you do not need run it IMO.
I install over the top as if I get an issue with the Beta's I simply "Roll Back" using the Device Manager.
This matters more to me as I use a 24" CRT so I have it all configured, and this method retains my settings, if it was an LCD it would not really matter.