i would go for the pc133 cas3, cause if its kingston, it should EASILY do cas2........and possibly even as high as o lets say 140mhz mebbe....oh wait.......you say duron? then DEFINITELY get the pc133, cause if you get the pc100 you wont be able to overclock it too much cause via boards dont run too stable past 110mhz fsb, which is only about +3mhz pci, so your pc133 should be able to do:
cas2 @ (hostclock+pciclock) 100mhz+36mhz=136mhz.......good enough.....