A motherboard chipset manufactured by Via. Gives the advantages of running an asynchronous memory-to-CPU bus in an AMD Athlon platform (meaning that the CPU bus runs at 100 MHz but the memory bus runs at 133 MHz, giving you a 3/4 clock stepping between the memory and CPU buses.) This feature enables an Athlon user the ability to run his or her memory 33% faster than AMD 751-based Athlon motherboards. Also, the KX133 is the only Athlon chipset (at the time of this WU) that supports AGP 4X, enabling twice the bandwidth between the memory and supporting video cards, and also gives a 'fast write' feature which increases the rate at which textures and geometry information are sent to the video card.

AMD Athlon motherboards that utilize the KX133 chipset at the time of this writing:
AsusTek K7V
Abit KA7-100