Mat,
The SE6 defaults to the lowest sync, 100mhz if any of the two are 100.
I don't know if you can unsync the two. Even if you do, you will attain a small advantage because you probably will still have to set your cas/ras settings to 333, where your real performance gain is achieved.
edit: Sorry, Invalid comment. I am speaking of memory, not processor/memory. That I don't know about as I have both 133.