Memory Module Speed
When you replace a failed memory module or install a new module as an upgrade, you typically must install a module of the same type and speed as the others in the system. You can substitute a module with a different (faster) speed but only if the replacement module's speed is equal to or faster than that of the other modules in the system.
Some people have had problems when "mixing" modules of different speeds. With the wide variety of motherboards, chipsets, and memory types, few ironclad rules exist. When in doubt as to which speed module to install in your system, consult the motherboard documentation for more information.
Substituting faster memory of the same type doesn't result in improved performance if the system still operates the memory at the same speed. Systems that use DIMMs or RIMMs can read the speed and timing features of the module from a special SPD ROM installed on the module and then set chipset (memory controller) timing accordingly. In these systems, you might see an increase in performance by installing faster modules, to the limit of what the chipset will support.
To place more emphasis on timing and reliability, there are Intel and JEDEC standards governing memory types that require certain levels of performance. These standards certify that memory modules perform within Intel's timing and performance guidelines.
The same common symptoms result when the system memory has failed or is simply not fast enough for the system's timing. The usual symptoms are frequent parity check errors or a system that does not operate at all. The POST might report errors, too. If you're unsure of which chips to buy for your system, contact the system manufacturer or a reputable chip supplier.
See "Parity Checking," p. 415 (this chapter).