Not much point in higher clocked ram with Sandy Bridge since OCing is done by increasing multiplier not Bclk. Save your money.
But see this before you make up your mind: http://www.xbitlabs.com/articles/memory/display/sandy-bridge-ddr3_7.html#sect0. You don't need fast memory to overclock, but you might need it to get the most out of your overclock.
