FSB, Memory Bus, DDR, Dual Channel

Source
My FSB is 200 MHz (my motherboard support 800 MHz)...
And my bus speed is 800 MHz!
What is the difference here?
Why only 200 MHz when I check it?

Specs for the CPU:
Intel Pentium4 2.6 GHz / Hyper Threading
Core: Northwood
Frequency: 2.6GHz
FSB: 800MHz
Cache: L1 12KB+8KB; L2 512KB

Specs for the Mobo:
Model: 865PE Neo2-LS
Supported CPU: Socket 478 Pentium 4 Processors(Hyper-Threading)
Chipset: Intel 865PE + ICH5
FSB: 800/533/400MHz
RAM: 4x DIMM support Dual Channel DDR 400/333/266 Max 4GB

Answer:
You are using DDR PC 3200 memory, also called 400 Mhz DDR
The memory bus [from memory to the MCH chip] runs at 200 MHz.
HOWEVER DDR (Double Data Rate) makes TWO reads|writes per clock cycle, once on the leading edge and once on the trailing edge, thus 2 X 200 = 400.

Then Dual Channel does the rest.
The mobo/chipset actually has TWO memory busses from the memory to the MCH chip.
The MCH stages these two memory busses to the CPU such that the CPU is getting FOUR read/writes per clock cycle.
Thus 2 X 400 = 800.

Of course you must have two memory sticks of same size et al, one in the even slot and one in the odd slot so that both memory busses are being fed from memory.

MCH [formerly Northbridge] is Memory Controller Hub
ICH [formerly Southbridge] is I/O Controller Hub.
Substantially improved from original NorthBridge and SouthBridge.
The interface between the ICH and MCH is far faster than what existed in the NB/SB interface. Those chipsets were designed to remove as many readblocks as possible for all types of data transfers.
BTW the AMD 64 bit CPU's have the MCH on the CPU chip.
The lower priced AMD MCH does NOT handle Dual Channel, only the higher priced version has a builtin MCH that handles Dual Channel.