Memory Help Pages

3. I want to buy some dimms to upgrade the memory on my PC but I am unsure of whether to get PC66, PC100, or PC133. Which should I buy to upgrade my system?

The first factor to consider in deciding which memory to use is the bus speed of the processor. To decide on this it is best to check the motherboard settings but there is a general guide below

66MHz (some are less than 66 but this no problem as far as ram is concerned)
Intel Pentium - any speed
Intel Pentium II up to 333Mhz
AMD K5 AMD K6 - any speed
Cyrix MI - all speeds (Non MMX)
Cyrix MII - (speeds vary from 66 to 83MHz for those above 66 use PC100 ram)

100MHz (a few are slightly less)
Intel Pentium II 350 onwards
Intel Pentium III (almost all - some from 600Mhz up are 133)
AMD K6-2-3D (almost all - some are less but PC100 is ok for all of them)
Cyrix MII - (speeds vary from 66 to 83MHz for those above 66 use PC100 ram) AMD Athlon - (almost all - some 133 Mhz)

133MHz
Intel Pentium III (some from 600Mhz up are 133)
AMD Athlon - (some 133 Mhz)

Once you know the bus speed of the processor then simply choose the ram of the same speed. However, It can sometimes be difficult to get the ram specification you want as it is may be out of stock. In this case it is always acceptable (as far as the processor is concerned) to use a faster specification Ram. The only problem is that the technology of ram chips has changed and older motherboards will not always recognise the newer technology ram. Additionally to complicate the matter even further there are cases where certain makes of ram will not run on certain motherboards though they will run faultlessly on others and some motherboards will not run correctly if you mix ram specifications. For this reason it is recommended that whenever you buy SDram Dimms you only do so from someone who agrees to take them back if they won't work in your system regardless of whether they are faulty or not.