You are getting it a bit mixed up. The 400 and 533 bus speeds are between the processor and the MCH. Internally the processor runs at the multiplier times 100 or 133 depending on the chip. This one for instance runs 19*133 to get 2.53Ghz. Much the same for the Athlon except that processor runs 266 between the processor and northbridge, and DDR memory being 100 or 133mhz but using the rising and falling edges of a cycle to make its effective bandwidth 200 or 266mhz (P4 bus uses the rising, falling, and in-between edges making it 4X instead of 2X.)
The easy way to explain overclocking of a P4 is that you just raise its bus speed to what you wish, and since the multiplier is locked at a certain number, it will always be that number times whatever speed you set it at. For instance, 3.0ghz on this chip you would set the bus to 158mhz (158*19=3002). Of course, if you change the bus speed then the agp/pci busses will be out of sync unless its on a board that can lock those (too tired to explain this aspect). It may want more voltage, it may want less heat, it may want lower fsb settings, or better memory, or it may want a lot of things, but Im sure there are guides that go more into it than this.