Correct me if Im wrong but running the RAM at slower speeds than the FSB will mean the FSB has to wait till the RAM is effectively ready (refreshed) for the next lot of data and thus cause a Bottleneck at a 5:4 Ratio, but if you run the RAM at 4:5 (which only Intel will give any differences) you may actually get a little more performance.
"This solution, however, isn't ideal. Running the FSB and RAM with a ratio causes gaps in between the time that the FSB can communicate with the RAM. This causes slowdowns that wouldn't be there if the RAM and the FSB were running at the same speed. If you want the most speed out of your system, using an FSB:RAM ratio wouldn't be the best solution."
Overclocking Guide - MajorGeeks Support Forums
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