The bus frequency and memory frequency are two different things. The manual is specific in saying that the memory frequency supported is 667 MHz, so I'd stick with that, as faster memory might not work properly.
The Dell website shows the 800 MHz memory as the first option for upgrading but it is more expensive than the 667 MHz memory. If it doesn't improve the laptop performance I see no reason to pay extra but I want to be sure.
Thanks for any help.