The front side bus (FSB) is the connection between the processor and memory. It's speed is expressed in MHz. The higher the MHz, the faster the FSB. The maximum speed of the FSB is a characteristic of the motherboard.

For SDRAM
100 FSB processor should be used with PC100
133 FSB processor should be used with PC133

For DDR,
200 FSB processor should be used with PC1600 RAM
266 FSB processor should be used with PC2100 RAM
333 FSB processor should be used with PC2700 RAM
400 FSB processor should be used with PC3200 RAM

FOR RAMBUS

400 FSB (up to 2GHz) Processor can use PC800 45ns or 45ns RAMBUS memory
533 FSB (from 2.1GHz) Processor can only use PC800 40ns and PC1066 RAMBUS memory

The newest DDR memory is PC3200. You should be aware that the JDEC standard for PC3200 RAM has not been finalized, so not all PC3200 memory will work with all motherboards and processors that support 400 FSB.