The default speed for the memory on the i7-900 series is 1066. Overclocking the memory on its own is a bit unorthodox, and I'm not really sure that it's worth doing TBH. It would be better to leave the memory multiplier be, and simply increase the main clock, which will then overclock the CPU and memory together. You should be able to get a worthwhile O/C on stock voltages - I got 15% out of mine without any effort at all.