No physical difference

You are right in that the difference between a 200 MHz and 266 MHz Athlon is like the difference between a 100 MHz and 133 MHz Intel chip. For instance, a 1 GHz T-Bird can either be specified to run at 200*5 (or 100*10 depending on how you look at it), or 266*3.75 (or 133*7.5). Based purely on the specified FSB, the only thing you can tell is what multiplier the chip is set for at the factory.

If you unlock the chip, you should be able to lower the multiplier and raise the bus speed. The only problem here is that some motherboards that use a SoftMenu-type setup don't actually start up with the multiplier you set. They might start up with the original, factory-set multiplier until the BIOS changes it to your setting. This is why some people are reporting problems running 200 MHz chips at 266 MHz bus.

Example: Jim sets his 1.2 GHz T-Bird (200 MHz) to run at 266*4.5 (or, again, 133*9) for 1.2 GHz, even though the factory specified the chip to run at 200*6 (100*12). When Jim turns on his softmenu-type system, the motherboard is set at 266 MHz FSB, and tries to run the chip at a multiplier of 6, for 1.6 GHz. The chip simply cannot run that speed, so the motherboard never gets to the point where it applies Jim's multiplier setting of 4.5.

If the chip can run at the higher speed just long enough for the softmenu mobo to change the multiplier to the lower value, the system boots normally.

If the multiplier is set in hardware, with jumpers or dip switches, the motherboard simply starts with the user-set multiplier, and everything should be happy.

All of these references to FSB actually mean the speed of communication between the processor and the chipset, NOT the speed of the RAM. The original KT133 is capable of running SDRAM at 133 MHz, but it cannot run the FSB (between chipset and processor) at 133 DDR/266 MHz.

I believe both the AMD and Ali DDR chipsets run the memory and FSB at the same speed, and the Via DDR chipset allows one of 3 scenarios:

both FSB and memory at 133 DDR/266 MHz

both FSB and memory at 100 DDR/200 MHz

FSB at 133 DDR/266, and memory at 100 DDR/200

I don't think the Via chipset allows the memory to be set faster than the FSB. I might have this reversed, but I think I'm right, since it's easier to get a 266 MHz T-Bird than it is to get PC2100 (DDRRAM spec'ed for 266 MHz).

Hopefully this helps to clarify instead of perpetuate confusion.