To install more memory in your PC to make it faster is, seemingly, an extremely simple task: all you have to do is to buy the memory in the store and install it on your PC motherboard, while its power is off, obviously.
Only that the incompatibilities between old motherboards and newer memory modules are more and more common: you buy the module, install it in your computer and the motherboard does not recognize the whole capacity of the module (for instance, you buy a 128 MB memory module and your computer only sees it as if it were a 64 MB one).
There are two limits in upgrading your memory. The first one is the maximum capacity of each module that the motherboard accepts. This limit is specified in the board manual. For instance, if your motherboard only accepts modules up to 64 MB of capacity, there is not point in trying to install modules of a greater capacity, as the motherboard will not accept them. Older motherboards (Socket 7, that is, for Pentium, Pentium MMX, 6x86, 6x86MX, MII, K5 and K6 processors) are limited to modules of 64 MB each. As of the motherboards for the Super 7 processors (K6-2 and K6-III), those accept memory modules of up to 256 MB each.
If you do not have the motherboard manual anymore to check the maximum quantity of memory that each memory socket of the motherboard accepts (64 MB, 256 MB, 512 MB, etc), all you have to do is to use the Sandra program (http://www.sisoftware.co.uk/index.html?dir=dload&location=sware_dl_3264&langx=en&a=). Execute the program and click on the Mainboard Information icon. In the window that will be exhibited, pay attention to the item "System Memory Controller". The number of memory sockets your PC has (Number of Memory Slots) and the maximum quantity of memory that your PC accepts (Maximum Installable Memory) will be exhibited in it. All you have to do is to divide the maximum quantity of memory by the number of memory sockets to know the maximum memory module that your personal computer accepts. For instance, in a PC where the maximum memory capacity is 768 MB and which has three memory sockets, that means it accepts modules up to 256 MB.
The second limit refers to the density of each memory chip. It is in that limit that the biggest compatibility problems are nowadays. Each memory module has a series of RAM memory chips. By dividing the capacity of the memory module by the quantity of chips it has, we find the memory capacity that each chip stores, in MB. For instance, in a 256 MB memory module with 16 chips, each chip stores 16 MB (256 MB / 16). However, the most usual unit to represent the density of each memory chip is the megabit, not the megabyte. Since one megabyte corresponds to eight megabits, all you have to do is multiply the value you had by eight to have the result in megabits. In other words, that memory module has a density of 128 Mbits per chip.
The whole problem is the following: motherboards that accept modules of up to 256 MB only accept memory chips with a density of up to 128 Mbit.
If we install the 256 MB memory module with 128 Mbit chip density from our example in a K6-2, it will be accepted without any problems.
However, suppose a 128 MB memory module using four chips. The density of that module will be 256 Mbits (128 MB / 4 x 8) per chip. Modules with 256 Mbit chips are only accepted on motherboards in which the capacity limit of each memory module is of at least 512 MB. In other words, that 128 MB memory module will not be recognized in a K6-2, for instance, even though the motherboard of that PC allows the installation memory modules of up to 256 MB. Even though the motherboard seemingly accepts the module capacity (128 MB), the problem is that it does not recognize the density of the chips used (256 Mbit).
In that case, that motherboard will only accept 128 MB modules with at least eight chips and 256 MB modules with at least 16 chips.