M6500 8gb memory not running at 1600Mhz

In the dell service manual, it reads the following:

"NOTE: Intel Core i7-920XM Quad-Core
processor supports memory frequency up
to 1600 MHz. All other Core i7 Quad-Core
processors support memory frequency up
to 1333 MHz only, even if memory
modules of higher frequency are installed."

According to the Intel Processor Identification Utility, my CPU is :

'Intel(R) Core(TM) i7 920XM Extreme Edition CPU @ 2.00GHZ

Also reports the IMC Speed is 1333Mhz

I recently purchased 4 8GB dimms spec'd at 1600MHz and installed them but they're all running at 1333MHZ.

Do I have to do something to make them run at 1600Mhz?