The VIA KT400A is the successor to the KT400, using a "spec" DDR400 part that is not present on the KT400. Also, the 400A uses what it calls "FastStream Technology" which supposedly helps the 64 bit memory controller to outperform the NForce2 chipset, as stated in the first link. Here are those links that review the 400A boards and shed some light on your question. Note that in these links, there are some comparisons between the two versions.
Carrie: Someone\'s definition of what constitutes cheating is in direct proportion to how much they themselves want to cheat.
Miranda: That\'s moral relativism!
Carrie: I prefer to think of it as quantum cheating.