PC100 is SDRAM
SDRAM comes in 3 different flavors
PC66 - cheap stuff for celerons
PC100 - for non-overclocking BXBoards
PC133 - for 133 MHz bus boards or people who want to overclock
Recently, someone mentioned PC150, but I havent seen it for sale
If you want some performance headroom, go for the PC133. Its good value for the money
Avoid RAMBUS at all costs!