Some 64-bit cards are backwards compatible, if the card and the slot have the right notches. Not only do you have to be look at the frequency and bit-width, but whether it's 3.3V or 5V. You should look at the manual or FAQ for the card on the manufacturer's site, if you're trying to be sure before you buy one. If you put a card in that's the right voltage but isn't backward compatible, it just won't work.