In a 1366 system, the default memory multiplier is 8, default CPU multiplier is 20 . So here's what ya start with say w/ a 920:
133 BCLK x 20 CPU Multi = 2667 MHz (2.67 GHz)
133 BCLK x 8 Memory Multi = 1066 MHz
I think if you check the SPD info w/ CPU-z, I think you will find that it's 1.65 at 1600 but only 1.5 at 1333 and 1066. Note that it may work just fine at 1.5 volts but it s only guaranteed to be stable at 1600 at 1.65 volts.
Now when it gets interesting is when you start overclocking. By setting the RAM at 1600, you now have this:
133 BCLK x 20 CPU Multi = 2667 MHz (2.67 GHz)
133 BCLK x 12 Memory Multi = 1600 MHz
Another option would be:
160 BCLK x 20 CPU Multi = 3200 MHz (3.20 GHz)
160BCLK x 10 Memory Multi = 1600 MHz
Another option w/ a good cpu cooler would be:
200 BCLK x 20 CPU Multi = 4000 MHz (4.00 GHz)
200BCLK x 8 Memory Multi = 1600 MHz
Now with regard to voltage, you want to generally go thru the same step of voltage optimization as you would on the CPU after settling on your PC. On my son's 920 for example, we wer abl to drop the CPU down from 1.25 volts to 1.1125 volts which also dropped Prim 95 load temps from 70-72 to the mid to hi 50's. Memory voltage was dropped from 1.65 to 1.5625 but lower than that OC became unstable.
If ya want a good guide to the voltage optimization procedure and OC'ing.
http://vip.asus.com/forum/view.aspx?id=20081220191040237&board_id=1&model=P6T+Deluxe&page=1&SLanguage=en-us