|DDR1-333 runs at 166MHz but can also be referred to by it's effective speed of 333MHz.|
DDR2-667 runs at 333MHz but can also be referred to by it's effective speed of 667MHz.
DDR1 & DDR2 are NOT interchangeable. They are notched differently so that they can not be installed in the wrong slot type. I'm not sure if you're asking about DDR1 or DDR2 RAM?