If you’re planning to add more RAM to your existing setup, you need to make sure that they will work together. Here are a few things to be aware of:


  • SIMMs are harder to match up. For example, you generally can’t mix a pair of Fast Page Mode (FPM) SIMMs with a pair of EDO SIMMs, although some boards will support mixing different types. Under no circumstances, however, can you mix types within a bank.You also need to pay attention to the speed. If your system has 60 nano-second (ns) access time SIMMs installed, and you add a pair of 70ns SIMMs you could run into trouble if your system needs at least 60 ns. Be sure to get at least the same speed or faster. Consult your system documentation to be sure. If your system accepts the mixed speed modules, the speed they will run at will be the lowest common denominator, e.g. if you mix 60ns with 70ns SIMMs, they will run at the slower 70ns.

    Also pay attention to the order. Install the newer and bigger pair into the first bank (bank 0) and move the older pair to the second bank (bank 1).


  • DIMMs are easier to match up. For example, you might have a stick of 64MB PC-100 Non-ECC RAM and add a stick of 64MB PC-133 ECC RAM. They normally work together, but please be aware that they will run at the lowest common denominator, in this case PC-100 Non-ECC mode.DIMMs are usually labeled with the bus speed they’re guaranteed to run at. PC-66, the first generation DIMMs, were made to run at a front side bus (FSB) speed of 66 MHz, because the processors they were used for (older Pentium II and Celeron) ran with a 66 MHz FSB. PC-100 means the DIMM can run fine with a FSB up to 100 MHz (or lower, they are backwards compatible). PC-133 means the DIMM can run fine with a FSB up to 133 MHz (or lower, they are backwards compatible). But the other way around you’ll run into problems when trying to run a PC-100 DIMM on a 133 MHz FSB.

    Then there is CAS (Column Address Strobe) Latency – short: CL. It describes the amount of clock cycles you have to wait for initial data access on the actual memory chip. The lower the latency, the faster the access. Therefore CL 2 would be faster than CL 3. However, since three clock cycles wait translates to about 24 nanosecond (ns) wait, compared to 16 ns wait with a CL 2 rating (assuming a speed of 8 ns), we’re talking a difference of 8 ns – a difference that is negligible for the average user. CAS Latency is more important when you start pushing the limits of a machine, e.g. when overclocking. Otherwise it’s probably not worth spending the extra money.

    The main thing to remember with CL is that your memory can only run at the lowest common denominator, so if you have a DIMM rated as CL 2 and a DIMM rated as CL 3, they will both run at CL 3.

Where To Buy

Depending on what you’re looking for, you have several options.

If you want a piece of cheap, generic memory and are confident that it will work fine in your machine, then your best bet is probably to go to our Best Prices page, use the filter option on the right to narrow it down by RAM size, type, and speed, and find the best prices online.

If you prefer to get quality brand-name memory for a decent price, and/or need some help finding the right memory that’s guaranteed to work with your system, you should check out Crucial Technology’s web site. They offer good quality Micron memory at a good price. Search by memory type to find the standard RAM you want to get.

Another great feature on their site is the memory selector to make sure you get memory that is guaranteed to work with your PC. If you have a brand-name PC that requires a very specific type of memory, chances are you can’t find it in stores, and that the manufacturer charges you an arm and a leg for it. But with Crucial’s memory selector you simply select the manufacturer of your PC and the model to get a list of all the memory that works with it – guaranteed.


Now that you’re armed with all that knowledge, you should be able to confidently get the right memory upgrade for your system. As long as you do your homework and prepare for the upgrade with a little bit of research, upgrading your PC’s RAM should be easy.


– Alex –

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