IT'S part of American folklore that Benjamin Siegel hated his nickname. And for a very good reason. People didn't call him Bugsy out of affection, but because he was as crazy as the term suggests - a thug whose violent rage became legendary in an era that didn't lack for crime-related headlines.Along with craziness, moreover, Siegel had a hefty share of vanity. According to the screenplay for "Bugsy," he dreamed of Hollywood stardom and cultivated his personal fame in ways that made his camera-shy accomplices more than a little nervous. When his picture appeared in the paper, he cared less about the caption than the way his suntan showed up. The trouble with Siegel as a movie subject is that he's been on screen before - most recently in "Mobsters," a forgettable crime film that flopped a few months ago. "Bugsy" is a far more ambitious effort with a large budget, a two-hour-plus running time, and no less a star than Warren Beatty in the title role. It tries very hard, but in the end, it's more familiar than surprising. Focusing on the last part of Siegel's career - thus complementing "Mobsters," which showed him as a young hoodlum on the rise - the new picture starts with Bugsy heading west to run a new division of the crime empire he's established with Meyer Lansky, Lucky Luciano, and the rest of their mob. There he discovers Hollywood and a new girlfriend, Virginia Hill, who's as clever and ruthless as he is, albeit less crazy. The film gathers momentum when Bugsy comes up with the idea that made him famous: building a resort in the Mojave Desert, filling it with patrons attracted by legalized gambling, and waiting for Las Vegas to spring up around it, making him and his associates wealthy. Bugsy's dreams are more expansive than his business savvy, though, and soon the cost of the project soars out of control. Since it's financed by a bunch of very acquisitive crooks, this puts Bugsy in a ticklish position. It doesn't help when Virginia is charged with stealing from the till. If there's an earlier movie whose spirit hovers over "Bugsy," it's "Bonnie and Clyde," the 1967 hit that launched Mr. Beatty's career. The similarity comes not just from Beatty's repeated presence as a photogenic mobster, but from the verbal and visual humor that gives "Bugsy" much of its personality. Mingling graphic violence with absurdist wit is a tricky business, though, and few have accomplished it with the verve that made "Bonnie and Clyde" a landmark. Barry Levinson makes a game try in "Bugsy," but doesn't quite succeed. You appreciate its eagerness to entertain, and you chuckle at its more outlandish twists, as when Bugsy dreams up the idea of assassinating Mussolini and saving the world from fascism. The movie is so finely tuned, however, that it loses spontaneity. It's amusing when it wants to be hilarious, heavy when it wants to be spry - too freewheeling for a conventional thriller, yet not imaginative enough to be the revisionist epic that scree nwriter James Toback evidently wanted to create. This doesn't mean "Bugsy" is no fun. Beatty and Annette Bening charge their roles with energy, and the supporting cast includes such talents as Harvey Keitel, Joe Mantegna, Ben Kingsley, and Elliott Gould in sturdy performances. The screenplay treats Mr. Toback's usual themes of love, money, and power with more restraint than he normally shows, and Allen Daviau has contributed expert cinematography. Add this up and you have an engaging movie, which may generate laughs and gasps when audiences are in precisely the right mood. But its atmosphere and timing often seem a little off the beat. Unlike the bullet that finally dispatches Bugsy, it's a near miss.