bir - I have not used the G4, but I HAVE used Vista extensively with several front-ends. My advice to for the time being is do NOT use Vista if you plan to use either StreetDeck, Centrafuse, or Roadrunner. None of them function without issue on Vista. Vista is very cool, but not ready for our community yet (or maybe the other way around). Vista's SP1 is already being released next month (supposedly) so that may address a few things, but the rest will likely have to be addressed through updates of the front-ends.
As far as XP, I have XP Pro in the car, but I have XP Home in the house. I have Streetdeck on both, and there is literally no difference in performance of that nor other F/E's. I guess it depends on what you want to do outside of your front-end.