If you shoot RAW, the preview is actually an embedded JPEG, not the actual RAW. If the images on your computer have been corrupted (wherever they were), you won't be able to reverse the process by loading them back onto the card.
What you need to do:
1) shoot with one of your other cards and download normally. If the pictures show the same problem, the card is not at fault.
2) Download the pictures using a different USB port nd cable (if possible, a different reader, too). If the pictures are OK, but come through corrupted using your normal reader, cord and port, it's the reader, cable or port. Then test swapping just one of those components.
3) Use your card in a different camera. If the problem doesn't show up there, your camera is at fault.
Oh, and the latest 1D IIN firmware is 1.1.2. Are you running that? It provides >8GB support.