If Robert Downey Jr. is evidence of anything, it's that these Marvel movies can make careers. Zoe Saldana is certainly no stranger for blockbusters, having starred in both Avatar and Star Trek. She's never been much of a Box Office draw herself, as her star vehicle Columbiana didn't do particularly well. Maybe this'll change in the future, because she's in talks to star in the upcoming Guardians of the Galaxy as warrior Gamora.
Gamora seems to be the token female of the group. She's a green-skinned alien assassin who was originally raised by Thanos (he's the baddie you saw after the end credits of The Avengers). She turns on him after released his true colors, though.