You definitely want to stay in Ibiza City, not San Antonio (unless you primarily want to chill at the famous Cafes there, like Cafe del Mar and Mambo Cafe). Most of the clubs are centered near or around Ibiza City (i.e. Space, Pacha); some clubs lie in between Ibiza City and San Antonio (i.e. Privilege, Amnesia). Plus Ibiza City has all the famous beaches (i.e. Playa d'en Bossa). You don't need to rent a car, you can get around fine using the buses. I actually rented a motorcycle for a day just for the hell of it. As far as the dress code goes, anything casual (they prefer no jeans, but you can get away with it). T-shirts are fine also. Just make sure they're stylish clothes.
