You probably can't overclock your memory at all. You can always turn down the multiplier to 8x or 6x and overclock from there. A i7 930 can get to 3.6-3.8ghz easily with litle voltage increase. 4.0ghz would typically require more voltage and becomes inefficient but most i7s should get there.
Should I be able to reach 3.2 or 3.4 GHz with my Ci7 930 using my Zalman CNPS9900 CPU cooler?