1066 isn't supported on all Mobos anyway, would be easier to tell if we knew what mainboard you have Anyway, I'd stick with what Deraldin said last.
What mkreku said is only relevant for Nehalem boards (X58 is a chipset supporting them), and there, the interesting thing really is the triple channel mode, and not ddr3, as only ddr3 1066 (which is slower than ddr2 1066) is officially supported. And for triple channel to be used you'd naturally have to have 3 or 6 sticks of memory. All in all, let's worry about these things once Nehalem, its mainboards and ddr3 are available at an affordable price