Bottlenecks aside, the 800 MHz is actually a quad pumped 200 MHz. So
for synchronicity using an equally fast RAM is recommended. The 667 is
a quadpumped 166 MHz. There would be some inefficiencies.

Another issue that you should look into is the bus width. Compare the
RAM and FSB bus widths. What really needs comparison is the total data
that can be sent by the RAM, and the total data than can be passed
through the Bus. Choose a configuration that optimises this. Getting
RAMs that exceed the FSBs capability would be a waste as would be the
converse.