Oops! My bad, I should really check things before I post.
The MCP seems to be the south bride, which also has integrated 5.1 audio.
The Ultra 400 is the north bride, which can give you double PC3200 performance, meaning 2x2x200 memory speed, giving you up to 6.4GB/s transfers. Of course with the appropiate memory sticks (high quality PC3200 RAM).
Note: The north bridge inter-connects the processor, memory and AGP bus (graphics card).
The south bridge takes care of everything else.
