As a professional in the invitation industry I can tell you I've seen it a hundred different ways. However, I do suggest sending out your STDs as soon as you are firm on the date and location of your wedding.
If you still want to send them out now and don't know the location simply state on your STDs that "invitations and traveling information will follow." That way they know they'll be traveling and they know to leave those days available.
Traditionally, rsvp information is sent with the invitation. If guests are traveling, you'll want to send all the invitations with the travel and rsvp info. about three to four months in advance. That's plenty of time for people to book and rsvp.
If you're worried about numbers then you can ask for a "soft commitment" on your Web site to get a pretty good idea.