As you can see above, on each notecard there is a space for the names of the adults/parents (what goes on the outer envelope) full names of any children (if applicable), mailing address and email. That way, when it comes time to write the invitations or send out a save the date, all of the information is right there. Each notecard represents an invitiation, so that you can count all of the notecards to know how many invites are needed. In the top right corner is the number of people that the notecard represents- this way, you can count all of the top right numbers to know how many people you are inviting. In the top left corner is the number of the actual invitation which you will note when you are physically addressing. You can put the same number on the back of the reply card so that you have a tracking system in case someone does not write their own name on the card and just checks x on the YES. On the bottom of the notecard, you can leave space for a gift (to fill in when that time comes) so that when you go to write thank-you notes, you have a note of what they gave you--- and guess what? The address and names are right there.
Wait, it gets better. So say you've written out all of your notecards and you have alphabetized them. What to do now? Where to put them? Pull out one of those old notecard boxes (for me, these were storing my old vocabulary cards, which I treasure- I'm weird). Make five dividers for yourself that read Invitations, Accepts, Regrets, Thank-You Notes To-Do and Thank-You Notes Done. That way, you can update the status of the cards each time you get a reply and each time you get a gift. You can also keep track of who you've written and who you still need to write.
Another variation on this system (which I didn't do) is to give different colored cards to different sets of people who have the addresses and contact info for those they want to invite- yellow to your parents, green to the groom's parents, pink to yourself and blue to the groom. Then you can combine, alphabetize and get rid of duplicates. A fine idea, indeed!
I, for one, think that this is an utterly brilliant system and have already implemented it. I'm definitely putting my notecard box in my carry-on when I travel back to DC!! Oh, organization!!