I answered on your other thread which is still open click here
After you swap over the drive with XP on it you would have the problems mentioned in the other thread and then you would have perhaps some problems with the upgrade to Vista. You might not it might run perfectly.
To be honest theres no way to tell if there would be any issues from the upgrade to Vista as the Vista scan my system for problems can only be run once your new PC with its new motherboard etc is up and running and assume there would be other new hardware as well.
And I would definately recommend backing everything up before you try. If you don't and things go wrong you will lose everything.