They say that if people do not RSVP within a week of the given date you should call them and politely say
"Hi (Jen) I was just wondering if you were wanting to attend the party I was having because I had not heard from you?"
People don't mind, people are mostly lazy about getting back about parties, its not rude and they will be happy you remembered them.
"RSVP is a must, will need final guest count "X"".....otherwise, depending on what kind of invite you are using, you can actually put a business sized card into the envelope that states, "Those who have not RSVP'd will not have room made, as resteraunt is strict on head count "X" days in advance"....my cousin did this for her wedding, and although some thought she was "haughty" b/c of it, she def got everyone to rsvp lol.....oh, and she also had written on her list of who had rsvp'd, and ran it against those who showed up, and her DF's friends were not above telling someone that even though they had an invitation, "space was limited, and therefore since they did not RSVP(nor were they marked down as an RSVP), there was not room for them...." lol, we def had fun with that.....