If the system supports 100MHz memory, you can normally use 133MHz memory in it as well, and can even mix the two. However, there would be no advantage gained as the 133MHz memory will only run at the bus speed of 100MHz. Using the 133MHz memory will not make the system more stable if it is being Over-clocked. Some systems, though, will balk at the incorrect speed being reported by the Serial Presence Detect program. We suggest using only the memory specified by the system manufacturer, and cannot guarantee that memory which is different than the manufacturer specifies will be 100% compatible.