Well it kind of depends on the brand of mobo. Diff brands have differing levels of bios support. Some boards even have differening levels of support depending on the board revision. Since you're talking original KT-133 chipset then you're limited to the 100mhz bus, so Duron 600-1.4Gig/100mhzbus T-Birds. If your particular board has good bios support then you might also be able to use the later 1.0-1.3g Morgan core durons as well.
You'll need the updated Kt-133A chipset & good bios support to make full use of the 133mhz cpus including AthlonXPs(Palominos, T-Bred-A & Bs), & the later 1.4-1.8gig Durons.
Give some more details & we should be able to tell you more...
Miah