To make the crust: In a bowl, combine graham cracker crumbs and butter until moistened. Press into bottom of an 8-inch springform pan. Cover, chill.
To make the filling: In a saucepan, sprinkle gelatin into milk; set stand for 5 minutes. Stir over low heat until gelatin is dissolved. Remove from heat; cool for 15 minutes.
In a bowl, beat cream cheeses until smooth. Beat in sour cream, Splenda and vanilla; slowly beat in gelatin mixture. Divide in half. Stir chocolate into one portion.
Spoon half of the chocolate filling into chilled crust; spread evenly. Carefully spoon half of the white filling into chocolate in small mounds. Using a narrow spatula or knife, swirl chocolate and while fillings, spreading each evenly. Swirl again. Cover and chill for 6 to 24 hours.
To serve, loosen cheesecake from pan; remove pan side. If desired, top with chocolate curls.