I have been using dual screens with my PowerBook, which has a Digital Video Interface (DVI) rather than VGA (whatever that stands for) the the external display. Doing a very quick A/B I did not see a huge difference between the two. Having one less adapter in my life is a Good Thing ™ so I always try to use DVI. I finally got around to Googling the difference and what I found is: DVI is best for LCD’s and the like, VGA is best for tubes. At least according to that guy. But he sure sounds like he knows what he is talking about.
It is sort of cool that our display technology has developed to the point that your display is showing you an exact copy of the digital information. CRT’s are soon to be a thing of the past.