Yep, got a mail from RAMBUS the other day, I asked them about the 659 chipset and if I could use my 16bit PC1066 RDRAM on it:
Thank you very much for your question that you submitted through Design & Reuse.com on SiS/Rambus joint development on the SiS659 chipset.
We are very excited about the development of this chipset as it will provide the highest performance memory subsystem for the most advanced Intel Pentium 4 Processors - 9.6GB/s of memory when used with 4 PC1200 Rambus(R) RDRAM modules. The chipset will also support slower grades of RDRAM memory.
As far as the actual memory module configuration that will be supported in actual PC motherboards, this will be up to the individual motherboard vendor. The chipset is being designed to support both 16-bit and 32-bit RDRAM modules. However, it will be up to the motherboard vendors to determine which type of memory modules they will build into their systems.
Please watch for more press announcements from Rambus, SiS and it's motherboard partners later in the year regarding specific product details.
I hope that this answers your question.
Business Development Manager, Rambus Inc.
So it's up to Asus whether I'd need to get 32 bit or not.