Depends on the motherboard. 650/680 chipset will let you overclock independently of ram speed. Core 2 Duo platform does not benefit from faster ram -
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However, with current mail in rebates the price difference between DDR2-800 and DDR2-667 is not that large. Either way I'd probably get DDR2 667 ram, E6420 and invest the rest into a faster graphics card.
If graphics card is not something you value, I'd probably go for faster cpu speed since it might allow you to overclock higher or result in less stress on FSB motherboard overclocking.
Finally, if you are looking at 965/975 chipset though, due to lowest 1:1 ratio, DDR2-800 is definitely recommended.