The new versions of the star destroyers are not Victory class. They are pictured in the new SW Insider, if someone explains to me how to post a pic, I'll scan and post it. The info Clai posted can be found in the Essential Guide to Vehicles and the Roleplaying games stats for ships. As far as the films go, they've never shown the so-called "Imperial Class" Stardestroyer because in all the pics of it in the Guide to Vehicles, it has a bunch of antennaes and crap on the command head piece. All the Star Destroyers in the movies look the same. Its just info they cooked up to add more dynamics to video and roleplaying games.
Anyway, the new ships do look much longer then they are wide, as opposed to the near equilladeral triangle of the AOTC troop transport. They have the groove down the sides that widen into the engine area very similar to the OT Destroyer. The head piece now runs perpendicular to what the OT Destroyer has. (it now runs lengthwise). There are insets about half way up the fuselage, one on each side. They also have a red stripe painted up the center of the fuselage. Its a very dull red, it looks subtle. In the same pics, there are also ships which strongly look like the rebel transports of ESB but they are clearly different ships. Older models perhaps. Again, tell me how to post pics and all put them all up, including the one of Obi-wan the jedi council member (as he now sits next to Ki-adi-mundi).
okay, they are already up on a site vanyoda pointed out to me.
Here's the battle, a "star destroyer" is seen at the top and another at bottom right.
http://users.pandora.be/famo/insider/h11.jpg
and because I said I'd post it, here's the council shot:
http://users.pandora.be/famo/insider/h10.jpg
Another major difference between Imperial Class and Victory class Star Destroyer's are that Victory's can actually land ON planets...
in A New Hope, the two Star Destroyer's pursuing the Millenium Falcon at Tatooine are Victory Class. As you can see, they look the same as Imperial class.
and Victory class Star Destroyer's were made near the end of the Clone Wars, so im pretty sure they'll be in Episode III. We'll probably also see Dreadnaughts, but any other light cruiser like the Dreadnaught (ie Carrack or Lancer) were developed in the days of the Empire, so we most likely wont see it unless Lucas doesnt stick to EU.
