Intel-brand boards tend to be more standards-compliant and overclocking-unfriendly. On the face of it, the warning means that you cannot run memory at DDR2-800 CL4 or CL3, or at different voltages than 1.8V. I would think DDR2-667 of any latency should be fine, but check with the vendor or manufacturer to be sure. Also check with the vendor or manufacturer about memory voltages other than the standard 1.8V.
What are your goals? If they include overclocking, don't get this board.