I like the movie 300, I feel at least captured the "essence" of the Battle of Thermopylae. As a historian, however, I have deliberately overlooked many of its... creative liberties, shall we say? If you want a good telling of the Persian War, may I suggest reading the words from the Father of the History of Western Civilization: Herodotus. The new annotated version by Robert B. Strassler is a good choice.
You can listen to the 300 Movie Soundtrack while you do it. But I also recommend playing the music your D&D games at certain key points. Why do I say key points? Because most movie soundtracks are composed to go along with what happens in the movie. 300 isn't that great for background ambiance, in my opinion. Most of the tracks are less than three minutes, not ideal for say, a prolonged combat. Still, you get 25 tracks to chose from.
Here are a few highlights (no, I'm not going to recommend "To Victory," since you've probably heard it many times):
"Returns a King" gets played with young King Leonidas returns victorious from the wolf hunt. You can use this after a similar, and yet "manly" victory you players might have, like defeating a great beast, for defeating an opponent in gladiatorial arena.
I've played it to announce when powerful warrior nobles, knights, walk into the room, since the music so obviously resonates the cheers and fanfare of the heroes return.
"What must a king do?" is only 1:06 long, but it conveys such powerful emotions. Yes, in the movie the king and queen are making love, but you can use this music for other things, like when characters have make a hard decision--as long as its epic. It's also a good transition piece to play between longer songs, or scenes in the game.
"Xerxes Tent" is the best song out there for representing the corrupt and powerful seducing somebody--the snake with the fruit, a rich patron wanting to hire the PCs to do something for an insane amount of money, a political fundraiser, and so on. The song also resonates the hint of betrayal, abandoning one's humanity and loyalties to himself and his friends.
Perhaps it comes as no surprise, but "Message for the Queen" is the most sorrowful of all the tracks. excellent for the death or remembrance of a hero or longtime companion. I'll say no more and let the song convey the message itself.