They have less unsprung weight which lets your springs and dampers work more efficiently, and they also tend to be a much larger diameter and thus stiffer. Those are both good things. However, if that were the only change on a 30+ year old bike, it would probably expose other weaknesses in the frame design.
They look really cool but unless you're a fab-master and you have plans to address the rest of the bike (frame, rear suspension, etc), I'd skip it and just buy some nice tires.
Starter thread about the current bike-of-the-month (see banner ^^^^^) which has inverted forks:
http://dotheton.com/index.php?topic=10971.0