I dont think you quite understand.
There is fsb speed between CPU and northbridge, and there is memory speed between memory and northbridge.
Now, a Duron uses a fsb of 100 Mhz, but because of the DDR nature of the bus, it performs like a 200 Mhz bus. most chipsets support asynchronous memory access, meaning the memory bus can work at 133 Mhz, while the fsb is only 100 (x2). It seems obvious that even a 133 Mhz memory bus cannot saturate a 100/200 Mhz-ish fsb, and will perform better then a 100 Mhz memory bus. THe remaining band with of the fsb (200-133=66 Mhz) is used for PCI and other stuff.
Also, you mention price.. actually, PC133 is cheaper than PC100 where I live. You would have to be nuts to buy PC100 now. PC100 or PC133 relates to the maximum rated operating speed of the module. Its true of course that a P133 DIMM will not work faster on a 100 Mhz bus (disregarding CAS latency)
Unfortunately, real world performance is somewhat different. It appears most chipsets perform much better when running fsb and memory busses in synch. Performance increases by about 10%, annihilating the advantage of the higher memory bus. IN other words, a Duron with a 100/200 fsb running @800 Mhz with a 133 Mhz memory bus, will be just about as fast as one with a 100 Mhz memory bus. However, running that same Duron @800 with a 133/266 Mhz fsb (6.5 multiplier) and 133 Mhz memory bus will perform about 10% better..somwhere between a Duron 850 and 900. Something to keep in mind when overclocking. Here is an extract from aces:
<i>
The biggest benefit of the VIAKT133A compared to the VIAKT133 is the ability to run the FSB in sync with the memory. <snip> Our measurements show that the running the FSB and memory in sync provides a 10% boost of performance in memory intensive games. Our loyal readers might remember that we explained a few months ago why running with a synchronous FSB and DRAM reduces access latency significantly. </i>
Read more about this here:
<A HREF="http://www.aceshardware.com/Spades/read.php?article_id=5000174" target="_new">http://www.aceshardware.com/Spades/read.php?article_id=5000174</A>
and here:
<A HREF="http://www.aceshardware.com/Spades/read.php?article_id=25000184" target="_new">http://www.aceshardware.com/Spades/read.php?article_id=25000184</A>
---- Owner of the only Dell computer with an AMD chip