I assume the MHz measurement of RAM is simply the bus speed of the motherboard (the speed at which the processor can access the RAM). Installing 400MHz RAM on a board that only supports a 266Mhz bus speed would yield zero difference in performance. The main bottlenecks on a PC are hard drive access times and bus speed. The more RAM you put in, the less Windows needs to access the hard drive for virtual memory, so that speeds it up though not necessarily that noticeably.