I'm going to let you in on a little secret: Twilight isn't going to be the hit that some people think it'll to be, and calling the thing a franchise before the first movie has even been released is premature to the point of hubris. No offense to women, but it's a vampire flick, of course it's going to appeal to them. But when have women ever driven a movie to blockbuster status? For that matter, when have men? Blockbusters have wide appeal from all demos, they don't make scratch from one over-driven segment. Young males may push it over the top -- not this movie I guarantee you -- but they can't do it alone, no demo can.
It costs pennies to have more scripts written from the rest of the books regardless of anything else, in comparison to what it costs to actually shoot the sequels. Everyone was talking about "His Dark Materials" and how all those books were going to become a franchise, but that movie didn't perform and all those plans for the next great franchise success went up in smoke.
Now, Twilight has some things going for and against it. Catherine Hardwicke is apparently a production designer turned Director with few actual directing gigs to her credit, and certainly nothing that stands out as successful. Melissa Rosenberg adapted the novel but this is her first (successful) venture into screenplays -- she's mostly a TV writer. I point this out because I think TV writers are probably better writers in general, but there's no telling if she could make the jump without stumbling nor do we have any idea of how well the translation from book to script went. Sometimes these questions give hope simply because it's new blood working on this thing, and that's the case here where even the cast is pretty undistinguished.
The upside of that is usually a cheap budget: $37 million for Twilight so it's not that hard to see it making back its costs, especially with the cheap advertising budget. The downside is that the movie isn't gonig to have a lot of buzz surrounding it with so few commercial time. On yet the other side of that which now confuses us, Twilight isn't opening against any significant competition and will debut pretty wide in over 3000 theaters.
If I had to guess, I'd say it opens first (a given) but with only a moderate take, and I predict this film will be added to the list of the next great franchise property...for direct-to-DVD.