Best headphones below US$100

hi people im a beginner and i want some advice to buy headphones , i'll use them for music , movies and gaming . i have been researching and a lot of people say the audio tecnica AD700 are good . by the way , could someone explain to me what dolby surround and dolby headphones do ? are they useful ? can i make use of them with onboard sound (asus p8z68 pro) ? will i need a sound card ? i'd like to know which pairs of headphones are good staying below US$100 .