In some recent tests I read about, the kt400 wasn't very impressive. I think it may have even had lower benchmarks than some kt333 systems because the memory can't be timed agressively at that speed. The same goes with the chipset itself ... it's timings don't seem very agreesive so you don't get much out of it.
Then you still have the issue of the memory bus being faster than the FSB, even after AMD bumps it up to 333.
I'd go with 333 myself (which is what I have actually.)