I spent some time looking into how Soft FSB works and I think I can explain how one reader was able to use Soft FSB to get additional bus speeds. If you go to IC Works website, it contains specs for all the clock frequency chips they make. Each chip is designed to support certain bus speeds, PCI dividers and AGP ratios – there is a spec sheet for each chip series which contains a table detailing this information.
For example ABIT’s BH6 uses W123 and the BX6-2 uses W164. If you look at the spec sheets for each, you will see what bus speeds are supported by the IC chip. Now, the data to enable these speeds is set by a serial data interface which is write-only to the chip. What the motherboard manufacturer can do is use all these settings or only a few, like 66 and 100. These are the settings you see in BIOS.
However, it appears that you can use Soft FSB to bypass the BIOS and enable all the settings which the chip supports. So if you have an ICW W123 chip and the BIOS only supports 66 and 100, you could specify the motherboard as the ABIT BH6 and the PLL Chip as the W123 and get BH6 bus speeds. You can not enable speeds that the particular chip does not support, so setting the motherboard as the BE6 for a W123 chip will not get you all the BE6 bus speeds.
CAUTION: I did this once and it worked, but this is a vary narrow experience base – proceed at your own risk!!