Here's the reference board from SiS with the 645 chipset and support for DDR333 (166 MHz double pumped).

We already reviewed the P4X266 some time ago; it will soon be replaced by the improved P4X266A, sporting a faster memory interface. Rumors claim that this interface is technologically very close to that found in products from VIA's competitor SiS. Another interesting product in this comparison is the SiS645 chipset, which is the only one that supports DDR-memory clocked at 166 MHz (333 MHz DDR), ensuring excellent performance.

VIA P4XB reference baoard. It uses the new P4X266A from VIA Technologies.