Who are the bigger movie stars? Al Pacino and Keanu Reeves or Jennifer Love Hewitt and Sarah Michelle Gellar? In a striking display of girl power, weekend moviegoers voted for TV’s Hewitt (Party of Five) and Gellar (Buffy the Vampire Slayer), propelling their modest, $16.7 million teen- slasher movie, I Know What You Did Last Summer, into the top spot. (It didn’t hurt that ads touted that the film was ”from the creator of Scream”; see story on page 6.) Pacino’s luxe, $60 million satanic thriller, The Devil’s Advocate, had to settle for a solid but still second-place $12.2 million debut. Another upset occurred in the middle of the list: David Duchovny (moonlighting from The X-Files) attracted less than $2 million on 1,479 screens with his docs-and-robbers actioner, Playing God; he was upstaged by British TV sensation Rowan Atkinson, whose Bean racked up $2.3 million on just 242 screens in Canada, after grossing more than $100 million overseas.