You know, cards might actually be a case where the original game is cheaper, more versatile, and more portable than the iPad.
Of course, the iPad is cooler. Maybe you can just open it up and use a note taking app to keep score.
No, I don't actually have anything terribly helpful to say. Try searching in the Apps Store for specific games you are interested in. I don't think you are going to have a whole lot of luck. Most card games depend on hiding your cards from your opponent, and that's kind of difficult and/or awkward with one device.
I believe there are several that will support playing with two or more devices either locally or through Game Center.