the P4 is Dual Channel and runs 400mhz and 400mhz
you can kind of think of it as full duplex
it is a 100mhz bus Quad pumped(multiplexed)
4x 100 = 400, FSB granted your using pc800
4x 100 = 400 BSB
the P4 has a seperate bus, called the 'RAMBUS'
thats also the name of the memory used for the 850chipset
and the AGP and PCI use dividers to control there speed
the 266 you are referring to is slower
what DDR did was Dual pipelined the standard 133 bus
1 of the reasons Rambus is faster than DDR is because when you use single channel pipelining, whatever is sent must reach its destination before anything else is sent
with Rambus, since it is dual channel, multiple instructions can be sent at the same time
to be honest with you, both RAMBUS and DDR are not very inovative in my opinion
just wait for holo graphic memory