Sorry, A card that is referred to as DDR 533 is running at 266MHz, but it's effective speed is 533 MHz because of the dual directional ram. They like to use larger number because it makes the memory seem faster.
The memory you want to get is DDR2 533 (266MHz).
If a card was referred to as DDR 1066 MHz (which I don't think exist). then, yes it would be running at 533 MHz.
As for the DDR2 - it's like an upgrade of DDR and the 2 is used to distinguish it as so.