In December it was reported that Eva had entered into negotiations for the role, but nothing official had come down. Now, both Yahoo Uk and Parda Phash are reporting that Eva Green has signed on for the female lead role of Artemisia. In actual history Artemisia was Xerxe's only female commander. She is best known in the history books for her strategical plans during The Battle of Salamis.

Former Bond girl Eva Green has signed on to play the scheming Artemisia in the 300 sequel, according to industry reports. - OMG Yahoo UK

London: The fair French actress Eva Green who played the bond girl in Casino Royale, has been signed on to play the female lead in in the sequel of artistically driven fantasy action film - 300. - Parda Phash

On February 3rd Black Book Magazine interviewed the actress and one of the subjects that came up was the rumor of her involvement in the project.

Is there any truth that you’re being pursued for the 300 sequel?
I don’t know, it’s a great character, so we’re trying to make her as complex as possible.

The Battle of Salamis

Artemisia counseled the Persian king Xerxes to coordinate a joint land-sea offensive. She wanted the Persian army to march to the Isthmus of Corinth and attack the Greek coalition that was fortifying there, while moving the fleet to attack the Greek triremes. In this, Artemisia hoped that the Greek ships would scatter to their city-states, leaving them vulnerable to an intact Persian fleet. Xerxes refused this plan, instead moving to attack the assembled Greek fleet at Salamis. Artemisia participated in the Battle of Salamis in September, 480 BC as a Persian ally commanding five ships.

Artemisia convinced Xerxes to retreat back to Asia Minor after the defeat at Salamis, contrary to the advice of Mardonius, who wanted Xerxes to stay. Xerxes then sent her to Ephesus to take care of his sons. In return, Artemisia's lands did well by their alliance with the Persians. - Wikipedia

At this time not much is known about the script. So, how much of the true events of Artemisia's life will actually be in the film is anyone's guess.