I'm going to be using 1066 with a 2.4/533. But I read about I think it was the Intel i850 chipset that doesn't require pairs of RDRAM as long as the mobo supports it. That way I can just get 256 in one stick and not have to toss 2x128 if I want to add more.
The Asus P4T533 i850E mobo supports 32 bit RDRAM. Make sure you buy 32 bit and not 16 bit. The 32 bit does not need to be used in pairs. Benchmarks show that it is a tab bit faster at accessing the memory than the P4T533-c which is 16 bit RDRAM.