Anandtech has posted a comparison between the SiS 648 and Via P4X400 chipsets using the (not yet official) DDR400 memory.

As proven by our benchmark results, DDR400 doesn’t exactly add a lot of extra performance to the Pentium 4 platform, in addition to not being fully operational with any of the motherboards we used (the SiS 648 Reference board is the exception). Again, this can probably be attributed to the fact that JEDEC has yet to approve an official specification for this memory type and lower latency DDR400 chips are not prevalent enough.

Full Review here