Yes, you can. The overall system will run at default 800. It's the safest route.
However, if you overclock to 1066, the overall system will run @1066. Set the voltage to the 800's memory. Maybe it's 1.9v, 2.0v, or 2.1v. Also, is this 800 RAM a value RAM? Directed more to price than performance? If yes, then I fear it cannot go over 800. Maybe slightly, but value RAM has little chance to go to its 'next' batch frequency (800 to 1066).