OC Workbench send words that SIS and VIA turns to limited support of DDR400:

The following is a translation of a report the Computer Weekly Magazine from Taiwan.

DDR400 is probably one of the most talked about during this year's Computex. SiS, VIA, Transcend, Twinmos all showcase related products based on DDR400. This didn't last long as there were no official DDR400 standards and reported compatibility issues. Chipset makers have thus removed all official information relating to DDR400 and it is left with support of DDR333/266/200 and AGP 8x.

VIA's P4X400 couldn't reach the performance level of DDR400 and have removed the information that P4X400 supporting DDR400. Last week's KT400 chipset document, the word supporting DDR400 has also disappeared. The same thing applies to SiS648, it is now only supporting DDR333 and DDR400 will only be supported in SiS648DX as described in the latest roadmap.

SiS mentions that from its test, SiS 648 can perform at DDR400 performance, JEDEC hasn't standardise the DDR400 standard. It might have some compatibility issues with certain brands of modules.