A 1066mhz FSB is actually quad pumped so it is truly 266mhz x 4. To get a true 1:1 ratio on you would use DDR2 533, as DDR means "double data rate" i.e. 266 x 2. As far as performance, it is best to use a 1:1 ratio but if you get a high enough clock speed on your RAM it will counter any performance hits that an asynchronous ratio might introduce. DDR2 800 will outperform DDR2 533 even though it isn't 1:1. Personally I underclock my RAM to 667mhz in order to run 1:1 with my overclocked cpu. In memory tests my RAM at DDR2 800 outscores it at DDR2 667. But in games and graphic benches the 667 @ 1:1 performs better.