Yeah, Gwen Stacy's in it, though apparently they're going to be having the main plot be about Spidey getting "the black suit" which, of course, winds up being the symbiote that creates Venom. I haven't read the comics, so I don't know if they're both a part of the same arc or not, but if not they're apparently tying them together somehow. Sandman is being played by Thomas Hayden Church, best known for his role in "Sideways." Topher Grace may be playing Venom; his role isn't confirmed, though Venom is. Sandman will be the main villain, though Harry Osborn will become either the new Green Goblin or Hobgoblin (I think just a new Green Goblin, though, probably with a new suit). There are also rumors of Black Cat, but I'm not sure these are true; I hope not. Too much to cram into a film, ya know? I don't think that there will be the Lizard, even though the second movie hinted at it--I haven't read anything about it at http://www.aintitcool.com/, that's for sure, and they post practically everything "Spidey 3" related.
Either way, I'm really looking forward to it. I do hope that there's less cheesy moments and screaming women than in the first two, though--I love them both, but think that there's a lot of really annoying brief moments. ("Go spidey go!"; "You kids don't play in the street anymore, okay?"; "What's that?" (as Dock Ock climbs up the building.) "EEEEEEEEK!" (Runs towards camera screaming.) Etc.)