We all hate that adding DDR3 sticks to a server slows down the QPI speed (or RAM Bus for lack of a better example).

That changes with the Nehalem EX proc (and perhaps Westmere), as the CPU governs the speed. You can throw up to 16 sticks of DDR3 RAM per CPU at either 800, 978, or 1066MHz, and the governing factor is the CPU:

Model	Speed	TDP Power	Cache	Cores	QPI Speed
X7560	2.26GHz	130W	24M	8	6.4GT/s
X7542	2.66GHz	130W	18M	6	5.86GT/s
X6550	2.00GHz	130W	18M	8	6.4GT/s
E7540	2.00GHz	105W	18M	6	6.4GT/s
E6540	2.00GHz	105W	18M	6	6.4GT/s
E6510	1.73GHz	105W	12M	4	4.8GT/s
L7555	1.86GHz	95W	24M	8	5.86GT/s
L7545	1.86GHz	95W	18M	6	5.86GT/s
E7520	1.86GHz	95W	18M	4	4.8GT/s

6.4GT/s – 1066MHz
5.86GT/s – 978MHz
4.8GT/s – 800MHz

As long as you stay with the 6.4GT/s CPU, you’ll be running at 1066MHz!