In newer systems using SDRAM or DDR SDRAM memory, you can use modules of different densities with no problem. For example, if your computer came with a 128MB memory module, you can add a 256MB module for a total of 384MB of RAM. However, if you have a "dual-channel" system and want to take advantage of that technology, you will need to ensure that the modules in each memory slot are the same density.

Although these are all types of double data rate (DDR) memory, the varying numbers refer to the different speeds of memory your computer was designed for. Crucial no longer carries PC2100 (DDR266) memory, but PC2700 (DDR333) memory and PC3200 (DDR400) are backward-compatible with PC2100.

Memory is designed to be backward-compatible, so generally speaking, you can safely add faster memory to a computer that was designed to run slower memory. However, your system will operate at the speed of the slowest module or system component (the "lowest common denominator" effect).

One thing to keep in mind is that the memory does need to be the same type-for example, SDRAM cannot be mixed with DDR, and DDR cannot be mixed with DDR2. We recommend that you use the Crucial Memory Advisor™ or System Scanner tools to find the right memory for your computer.

For more details about memory speeds, see Memory Speeds and Compatibility.

The terminology "dual-channel memory" is being misused by some in the memory industry, which can mislead the consumer. The fact is there's no such thing as dual-channel memory. There are, however, dual-channel platforms.

When properly used, the term "dual channel" refers to the DDR or DDR2 chipset on certain motherboards designed with two memory channels instead of one. The two channels handle memory-processing more efficiently by utilizing the theoretical bandwidth of the two modules, thus reducing system latencies, the timing delays that inherently occur with one memory module. For example, one controller reads and writes data while the second controller prepares for the next access, hence, eliminating the reset and setup delays that occur before one memory module can begin the read/write process all over again. Think of it like two relay runners. The first runner runs one leg while the second runner sets up and prepares to receive the baton smoothly and carry on the task at hand without delay. While performance gains from dual-channel chipsets aren't huge, they can increase bandwidth by as much as 10 percent. To those seeking to push the performance envelope, that 10 percent can be very important.

If you have a dual-channel platform and you want to take advantage of the performance gain it offers, our advice is to simply purchase your DDR or DDR2 memory in pairs. However, be very careful to order two modules with the exact same specifications; the modules must be identical to each other to perform correctly.

When adding new memory, you should match what is already in your system. Adding non-ECC memory to an ECC system will disable the error-checking and correcting ability of your memory modules. While your system may still operate, the enhanced features of the ECC modules will no longer be active in your computer.

You can determine if your system has ECC by simply counting the number of black memory chips on each module. ECC (and parity) memory modules have a chip count divisible by three or five. This extra chip detects if the data was correctly read or written by the memory module. If the data wasn't properly written, the extra chip will correct it in many cases (depending on the type of error). Non-ECC (also called non-parity) modules do not have this error-detecting feature. Any chip count not divisible by three or five indicates a non-parity memory module.

Using ECC decreases your computer's performance by about 2 percent. Current technology DRAM is very stable, and memory errors are rare, so unless you have a need for ECC, you are better served with non-parity (non-ECC) memory.

CAS latency (also referred to as latency) is the amount of time it takes for your memory to respond to a command. Specifically, it is the length of time between memory receiving a command to read data, and the first piece of data being sent from the memory.

Unless you are a computer enthusiast looking to push every last bit of performance out of your system, CAS latency needn't be a deciding factor when purchasing memory modules.

You may see CAS latency (clock latency) referred to as CAS#, CL#, CAS=# or CL=#. The performance difference between CAS latency ratings is virtually undetectable.