Even though it says it doesnot support that card, there is a good chance that the 800MHz card can get your system running. So when you plug the card it will automatically overclock it to 1066MHz. You cannot decrease the frequency but you can increase the timings of the ram to make the card more stable. Just make sure that the voltage is correctly set in the bios.
You can try this out even if it doesnot work it won't damage your motherboard or other components. It is always better to buy new ram cards.