kt400 was a step back from kt333 also
basically on P4 and athlon xp systems, the chipsets are split in to northbridge and southbridge
the northbridge connects the CPU to the RAM, AGP, PCi etc etc, the north bridge also connects to the southbridge (and thus south bridge to the cpu) but the southbridge typically runs less intensive tasks like USB ports, fire wire, and audio
here is an example of the P4 / athlon xp set up
ive drawn it so u can see the role played by the chipset. as u can see in the p4/ xp set ups the chipset is very much the hub of all communication
p4 and athlon xp set up
next i did an athlon 64 board, this is set much like the p4 and xp boards. the via chipset still employs a north bridge --> south bridge set up. as u can see the northbridge chipset is still very much the communication hub except on A64 the memory controller is on the cpu itself allowing it to directly access the memory with out goin thru the north bridge....this improves performance by reducing latency
a64 via boards
last but not least an nforce 3 board.....nvidia had the idea to combine the n/b and s/b on to one chip thus reducing latencies even further, but to be honest it makes no difference to reduce latency to ports like usb and firewire. again the memory and cpu communicate directly improving performance over the intel p4 set up
athlon 64 nforce board
this is a very simple description and hope its of use to you