To clear up the controversy, it is NOT a Sovreign class, that much is due to the apperant NX registry, and it's not a Nova varient, the saucer is all wrong for that.  The Registry number is NX-****-G, as the space between the "NX" and the "G" dictates that there are only four digits in the unknown number.  The hull is AT LEAST 400 meters long, and the picture doesn't show the nacelle length, so my best guess is that it is a Sorveign varient, although an entrely new class of ship, most likely somewhere between a heavy cruiser and a dreadnaught.  Another possiblity (And this is unlikely) is that this is actually a refit of the USS Sovreign, and some n00b tried to retexture it as the Enterprise-G, but forgot to change the NX in the registry.