For C2D, memory at DDR2-800 will improve performance over DDR2-667; paradoxically, running at DDR2-533 will likely improve performance over DDR2-667. Thus, if you get the DDR2-667, try running it at DDR2-533 speed.
That should probably make sense, but it doesn't. So let me ask in the shortest sentence possible.
And if your board supports only up to 667, do you get 800 and crank it down, or 533 and crank it up?