Buying into the post-Cannes buzz, Columbia is betting To Die For could actually live up to its title. Before being hailed as the festival's surprise American hit, Gus Van Sant's dark comedy was slated for a ''limited'' July release, where it would likely be swamped by its blockbuster competition. Columbia's lack of confidence was hardly surprising considering the director's last film, Even Cowgirls Get the Blues, scraped up a paltry $1.7 million. The studio is now holding the movie, which stars Nicole Kidman and Matt Dillon, for a nationwide September opening. The marketing plan probably shouldn't follow Van Sant's Cannes line: ''Cowgirls was as good as anything I've made. I feel exactly the same about Cowgirls as I do about To Die For.''