Scarlett Johansson

Scarlett burst into public prominence as a young teenage actress, first being noticed in the indie classic Welcome to the Dollhouse. Since then she's made over 20 films, likely being one of the few actors in history with as many films as her age, being born in 1984, so she's still only 24. Her most acclaimed and demanding part was as the friend of Bill Murray in Lost in Translation, when both were temporarily American fish out of water in Japan.
If anyone is going to be the new Marilyn, I nominate Scarlett, who has better acting skills and is just as beautiful, resembling the fabulous glamour queens of the silver screen. Currently a Woody Allen favorite, she's appeared in his films Match Point and Vicky Christina Barcelona.
Somehow, she manages to have both the "girl next door" and the classic voluptuous film siren qualities, which make her one of the most popular and alluring modern actresses.

Natalie Portman

Natalie was discovered on Long Island by a fashion modeling rep in a laundromat with her mom. When she showed up for a fashion shoot, she was sent to audition for Luc Besson's film The Professional. She got the lead part with no experience at age 12. She has since starred in many films, including the second Star Wars trilogy, and garnered an Oscar nomination for supporting actress for her daring role in Closer. As you can see from some of these photos, she would also be a great fashion model, and to me has some of the same graceful qualities as Audrey Hepburn.