Pictures from the bottom left: http://www.smitteninvitations.com/, second up: http://www.blueribbondesign.com/, top: http://www.smitteninvitations.com/, right: http://www.saimasaysdesign.com/.
It’s best to send a Save the Date card after you have reserved several of your most important vendors, and after your wedding venue(s) are set in stone. Don’t feel like you need to send them the day after you become engaged, six to nine months before the wedding date is considered ample time. There is no proper etiquette as to which form of announcement is better than the other, but always remember that early communication about your wedding helps guests prepare for the level of formality of your special day. Never send emails to your guests if you plan on having a black tie, only stationery will do. If you are having a casual wedding, a formal email to a handful of family can even be appropriate, but in doubt, send a card which is so much more fun to recieve anyhow.