There's a lot of flexibility here - both in the overall design and the details you might want to include on the back. As a reader, I love to collect things like this - just a few more insights into the characters or books I've come to love. As an author, it gives me a chance to express my vision of my characters in an unrestricted way.
Romance Trading Cards If you're getting cards made up for any of the conventions, be sure to stop by there and upload a few of them and add your name to the list so that readers will know you'll have them. (They are doing several auctions for charity as well, so it's always worth it to have something to go to a good cause.)
The only thing to keep in mind is that the cards are small - 2.5 x 3.5 inches, so a little larger than a business card. That can make it difficult design-wise. Too much info and it will get lost. I stuck with fairly simple stuff, myself - just a teaser quote and left it at that. Think I will start out with four designs to start with and add to them as the next book in the Brush of Darkness series continues.
You can see the fronts of all four cards on my website. I'll have the backs up there later this week. :)