I had the EXACT same problem as you. I got married 3/28/2007. GOODLUCK every bride on a budget comes across this problem.
I didn't word my invitation differently. Although I did add a RSVP to my invitation seperately in a different envelope already addressed, all they had to do was check yes or no, and put who was coming, (for seating chart purposes). I picked up the blank cards at Target or Walmart the RSVP cards, found a wording/font, look I wanted for them online and did them my self. To suit what I needed.
I also made sure to tell everyone, hey please rsvp, it's very important, that way you will have a seat... lol Made a joke out of it. I didn't put taht on my inviation though, just made a few phone calls after a couple weeks and if I didn't get the card I called to ask them personally, I just wanted to make sure you didn't loose the RSVP card, you won't be able to make it right? You'll even get those "10 maybes" "30 yes" and "9 nos" but when it comes time for the wedding 3/4 of the yes's actually show, 1/2 the maybes show, and even a few of the no's still show!
If you are catering, I suggest you order LESS that what you expect, b/c if you order more, you'll spend more, and if it doens't get ate, you don't get a refund. Not everyone will eat, depending on the time of your wedding. So goodluck.
I have a GREAT ideas for a wedding on a budget, that we did our selves that were a huge hit, so if you have myspace www.myspace.com/joelpole msg me, and I will send you pics and stuff of what we did to save thousands of dollars. Including the invitations/rsvps/favor's etc.