I was talking to one of BM's last night about these sorts of issues and she agrees that people usually wait until they get the formal invite to do anything, so they probably won't RSVP with the STD, however I have some family that will not even go to the website even though they have computers and everything, so we are sending traditional RSVP's hoping to get more that way, we are providing links on our site for people who would rather RSVP online, but I doubt many will.
As far as timing, we sent our STD's in January and will be sending out the Invites hopefully by the end of this month. If you send them too close together I don't think it has the same effectiveness, but if you send one or the other too early or too late it also looses that effectiveness.