Because AMD CPUs have an on-die memory controller, the speed of the RAM is directly affected by a divider in proportion to the FSB.
Take a look at this chart:
You're able to get by with the X2 3800+ and the DDR2-667. 2000MHz / 6 = 333MHz. Since DDR2 is double pumped: 333 x 2 = ~667MHz.
Your RAM will operate at its rated speed, but you won't get the same result with DDR2-533. You will however notice a performance gain if you were to go with DDR2-800. It would be able to run at its rated 400MHz.
Below I've recommended some choices. The lower the latency, the better .. but that usually means an increased price tag. Only if you can spare the cash ... otherwise you can get by with the DDR2-667.