cpu and motherboard fsb speeds

[roop]

Hi,
I have an old Gigabyte ga-7zx series with via kt133 chipset. Gigabyte only recommend using AMD Athlon cpu's with fsb 100 (doubled to 200Mhz) on their website with the latest bios. However, my Gigabyte manuel has a section on CPU setup which says the bus speed is selectable between 100 and 133 Mhz. I find this confusing, because surley I can set the speed to 133 Mhz and then use, for example, a cpu with fsb 133Mhz.
(Note1: It also says, however, the FSB Speed of the Via KT 133 is 100Mhz. Perhaps this is where the bottleneck is?)
(Note2: I am asking this question because I recently fried a cpu fsb 100 (incorrectly seated heatsink!!), and went out and bought a replacement cpu with fsb 133, which works, but at a slower speed.)

I hope all this makes sense, its a steep learning curve for non technical folks like me!