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!

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