DDR 1600 seems to be the price / performance sweet spot.
The 920 is the OC'ers dream as it's relatively cheap so if ya fry it, little lost. The "920 is just as good as a 975 cause you can OC a 920 to 975 speeds" argument doesn't fly tho as it ignores the fact that you can OC a 975 higher and easier than you can a 920.
Why 1600 ?
At 133.33 Base Clock (BCLK) and a memory Multiplier of 8, you reach that 1066 number, however....
At 133.33 Base Clock (BCLK) and a memory Multiplier of 12, you reach that 1600.
But that's really not where people are going.
Increasing your BCLK to 160, your CPU speed jumps from 2.67 to 3.2 GHz and w/ a MemMult of 10, hits DDR 1600 level
Increasing your BCLK to 180, your CPU speed jumps from 2.67 to 3.6 GHz and w/ a MemMult of 8, hits DDR 1440 level
Increasing your BCLK to 200, your CPU speed jumps from 2.67 to 4.0 GHz and w/ a MemMult of 8, hits DDR 1600 level
-w/o disabling anything and using a core voltage of 1.200 and DRAM voltage of 1.56, I am doing 3.7 Ghz w/ a BCLK of 176. Memory is at 1408 and Core temps are 66/63/63/63
-w/ speedstep and HT disabled and using a core voltage of 1.300 and DRAM voltage of 1.56, I am doing 4.2 Ghz w/ a BCLK of 200. Memory is at 1600 and Core temps are 74/71/70/70