Even in the old days of 66MHz fsb, overclocking to 75MHz worked every time, and in some cases 83MHz worked.
75Mhz put PCI at 37.5MHz, and AGP is 2x PCI.
You can use that as a given today as well, for the 100% safe maximum in most cases.
With the KT133
Using FSB speeds of 100 and up, and it's 1/3 for PCI,
so 100MHz fsb = 33/66MHz, and 112MHz fsb = 37.5/75MHz.
With the new KT133A
Using FSB speeds above 132MHz and it's 1/4 for PCI,
so 133MHz fsb = 33/66MHz, and 150MHz fsb = 37.5/75MHz.