MOVIES To heck with Tatooine. Hayden Christensen's going to Italy! ''I am going to work on a movie called The Decameron,'' says He Who Will Become Darth Vader. Costarring The O.C.'s Mischa Barton and based on Boccaccio's 14th-century classic, ''it's about a bunch of youths in the 1300s escaping the Black Plague.'' Sounds cheery! ''Actually, even though there's that plague plotline, it's a comedy. [We] get into adventure and mischief. It's going to be really funny.'' Especially watching Barton attempt medieval English. . . .Bryce Dallas Howard (The Village) is in negotiations to reteam with M. Night Shyamalan for Lady in the Water, about a guy (Paul Giamatti, the director hopes) who finds a sea nymph Howard in a swimming pool. Hey, that kinda sounds like 1984's Splash, which Bryce's daddy, Ron Howard, directed!. . .Deal Report exclusive! While setting up his next project, The 8th Voyage of Sinbad, at Sony, director Rob Cohen (XXX, July's Stealth) looked up famed visual-effects master Ray Harryhausen. ''I met with him and said, 'I really would be honored if you would be with me on this, since none of us would be here without you.' What Ray understood in movies like 1958's The 7th Voyage of Sinbad was how to do action- fantasy-adventure at a time when there was no CG. So he's coming out of retirement to be our creative consultant, our godfather.'' What convinced the 84-year-old veteran? Keanu Reeves, who will star as the titular hero, sailing to 10th-century, civil-war-torn China. ''He just went, 'Oh, that's so brilliant.' Ray thinks Keanu [could be] the greatest Sinbad ever.'' All heroes need a beautiful gal pal, and Cohen thinks Ziyi Zhang is the one for the job. Plus, he reminds us, ''Sinbad's got a crew, so it's gonna be a whole interesting group of actors. I've talked to Jet Li, Djimon Hounsou, and'' busy guy! ''Hayden Christensen.'' But the biggest surprise in this monster-filled action flick that begins shooting in January is the rating. ''I'm gonna make it PG,'' says the usually PG-13-friendly Cohen, ''so that everyone can go!''