Some people are commenting that the DDR2 667(or 675 there) sees better performance then the 800mhz memory due to the fsb on the newer boards not seeing a gain by going with the faster memory.
With a Core 2 there is a performance benefit with faster memory even though you are no longer running it 1:1 but I don't think it's worth the price of the faster RAM. However, if you were to use it for OC headroom then there is a definite advantage to it.
That's not always the case. I went from two 512s of Corsair xms series PC3200 dimms to a pair of Kingston 1gb Value Ram PC3200 dimms and actually have noticed a gain there. Far few errors are seen and even games run smoother without locks at high details even. There was no difference in speed there except the Corsair has a lower Cas Latency.