Preheat oven 325 deg.
Combine over hot (not boiling) water: chips & 1/2 c sugar.
Combine crumbs, sugar, and butter; pat into 9-inch springform pan (1-1/2" up sides).
Beat cream cheese until light & creamy, beat in 3/4 c sugar. Mix in sour cream and vanilla. Add eggs. Divide batter in half, stir melted chocolate into one half, pour into crumb-lined pan. Cover with plain batter, swirl with knife.
Bake at 325 deg, 50 minutes or until only a 2-3 inch portion in center will "jiggle" when shaken gently. Serve chilled.