On the Intel site, we can see that the memory types can be DDR3-800 or DDR3-1066, that maximum QPI speed is 4.8GT/s and that maximum bandwitch is 25.6 GB/s.
So without overclocking, what are the interest of DDR3-1333 and upper ?!
No, no, I'm well speaking about QPI (see my initial post); on the Intel webSite, the memory types is "clearly" indicated as DDR3-800 or DDR3-1066.
So what I understand is that without O/C, and without Turbo (I agree it would be idiot to NOT use it), it is limited to DDR3-1066.
Everyone agrees ?
Nope. The memory is unrelated to QPI, the memory controller is on the CPU itself. Originally, the memory multiplier was going to be locked, but Intel unlocked the memory controller and now you can access DDR3-1333, 1600, 1866, 2000, and even higher with any of the LGA1366 CPUs (although you will have to manually adjust the memory settings in the BIOS, including possibly memory controller voltage).
You definitely don't need to overclock your CPU core to use memory faster than 1066.