Generally, I've found that once you write over the card, your images are gone. However, I have seen stuff on there from months ago using Image Rescue and Rescue Pro. Viewing it was fine, recovering it was not. Sorry to be so pessimistic, I don't want to give you a false sense of hope. I hope that, at least, you hadn't written over the card.
Just as a general workflow note, i use 8 - 2 gig (or 4 gig) cards in rotation, numbered in order. That way if a card gets missed we can catch it within a few shots and hopefully we haven't come full circle to that card again.
Good luck.