Reports suggest that the KT400 chipset with DDR400 memory is actually slower than DDR333 memory - but I cannot confirm this.
It is also said that it does not properly support DDR400 memory. I can confirm this from personal experience. Apparently only a few manufacturers memory seem to work with this chipset, presumably the slower ones, which probably accounts for the first report.
Anyone thinking of using DDR400 memory with an inexpensive chipset would do better to look at the VIA KT400A.
Better still, go for the new nVidia chipset with 2 dimms as this allows for higher bandwidth (one dimm will be reading/writing data whilst the other is preparing etc).