Yes, a card. A letter probably will do fine too, except that people can leave pretty cards as decorations in their office, and letters are usually long-winded and formal. But, like you say, make certain it's a card with no text printed on the front or inside, and it had better be a quality card too (read: not the 99c kind). Get a couple opinions from people with good taste.

Write three or more sentences with a black rollerball (or better) pen, and make sure it's neat. You might want to practice it several times on normal paper first. Express your appreciation for the interview, without being obsequious, and wish them well. Sign it.

No matter what the outcome, it leaves a better opinion about you.