Don't do response cards at all. In addition to being an extra expense (card, envelope, and postage), they are a concession to the rude people of the world who don't respond to a RSVP. In my perfect world
I would like to think that anyone who is honored enough to be invited to my wedding will be able to pick up the phone and call me to let me know if they are coming or not. (Great in theory, but for my own wedding I had a few people who didn't respond, and when I called them, some of them actually said that they didn't know they had to RSVP because there wasn't a response card. Duh - that's what those little letters "RSVP" on the invitation stand for.)
I like the e-mail suggestion, and those who aren't computer-savvy are more likely to RSVP the old-fashioned way anyway.