Sorry, but you'll have to stick with 512MB DIMMs, since the KT133 supports only 6 Banks of RAM (3 double sided DIMMs) so with 256MB modules you'll be limited to a max of. 768MB. Of course, you might try with 2x256MB and 1x512MB module or you might want to wait for Microns MAMBA DDR chipset...
I recommend using an Abit KT7 (or KT7-Raid) and buying a Duron 600 or 650 overclocking it just by closing the gaps using a soft pencil and adjusting the bios to have it run at 1GHz! Mine is running rock stable (512MB PC133 Infineon, Abit KT7, Duron 600@1Ghz). The money you safe on the CPU is best invested in good quality memory.