Well, there is a "middle man" - that is the northbridge.
So, you have on one side of the northbridge the memory controller with access to memory. The memory is accessed at a frequency (133, 166 or 200MHz), double or single data rate (DDR in your case).
The memory is accessed in your case at 200 MHz. Commands are sent synchronously at this clock, while data is sent twice as fast (the essence of DDR). There are some delay times needed for the memory to get its data to the north bridge. Those are CAS Latency, RAS latency and some other. You can increase the memory frequency, but to have good results you must sometime increase the delay times. You can have a memory that is 2-2-2 at PC3200 but only 3-4-4 at PC 4400, let's say. Wheter the increased frequency with increased delays help, it's a not yet decided manner (usually they do, but not very much).
Now, the processor is on another connection. The processor has an external frequency, and a multiplier. While the communication on the processor Front Side Bus (its connection to the north bridge) is DDR for AMD AthlonXP and similar, the Pentium 4 processors use a schema that multiplies data speed by 4. This way you have the 200 (100MHz DDR), 333 (166 MHz DDR) and 400 (200MHz DDR) buses for AMD Athlon XP, and 400 (100MHz 4x), 533 (133MHz 4x), 800 (200MHz 4x) and 1066 (266MHz 4x) on Intel processors.
The FSB will be automagically detected if you have a mainboard new enough. If not, it is probably 166 MHz for the FSB. For memory, you could go with either PC3200 or 200DDR or 400, or with PC2700 or 166DDR or 333. The latter choice, while apparently slower, is better as it reduces the wait times (the FSB and memory are synchronised and usually commands can be transmitted faster). It is just like you going with a fast car, in heavy traffic of slow cars. You can sometime pass over the car in front of you, but sometime you must brake and pull back. If your car is just as fast as the others, you go easy with the flow of traffic.
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