Which Type of RAM Should You Buy to Upgrade Your PC? - dummies

Which Type of RAM Should You Buy to Upgrade Your PC?

By Mark L. Chambers

If you’re considering installing a new motherboard and CPU on an older PC, double-check that the new motherboard will use the same RAM type and speed as your current motherboard. Review the different types of RAM available for PCs made within the past five years or so.


Double data rate (DDR) modules were the first 168-pin standard Dual Inline Memory Modules (DIMMs) available for PCs; they’re commonly used on older Pentium and Athlon computers running Windows XP.

The double in the DDR name is significant because a DDR module effectively doubles the speed of the module, compared with older Synchronous Dynamic Random Access Memory (SDRAM) memory. Also, DDR memory is assigned a speed rating as part of its name, so it’s commonly listed as DDR266 or PC2100 (for the 133 MHz speed versions) and DDR333 or PC2700 (for the 166 MHz version).

As you might guess, the faster the access speed, the better the performance. The speed rating that you should choose is determined by the memory speeds that your motherboard supports. DDR memory modules have one notch on the connector and two notches on each side of the module.


DDR2 modules doubled the data transfer rate between RAM and motherboard, but they basically look the same. These days, DDR2 modules are most often found on lower-cost PCs running Windows Vista and Windows 7.


Here’s a good question: Which new memory specification is fast replacing DDR2? That’s right: DDR3 modules provide the best performance around, suitable for PCs running Windows 7 and 8. DDR3 modules were once significantly more expensive than their older DDR2 brethren, but DDR3 modules are now the most common type of RAM on mid-range and high-end PCs. DDR3 modules are available in capacities up to 8GB.


Rambus (RDRAM) modules are much faster (and also more expensive) than standard DDR modules. In fact, until the arrival of DDR2 and DDR3 memory, RDRAM was the memory standard of choice for high-speed PCs. DDR3 has now crept up from behind and taken the coveted title of First Place Performer, and RDRAM is slowly disappearing from the market.


SDRAM (sometimes called SyncDRAM) took the form of standard 168-pin DIMMs. These modules were standard equipment on most Pentium III and some older Pentium 4 machines. SyncDRAM ran at an access speed of 133 MHz, which is too doggone slow for today’s fast processors. SDRAM memory modules sport two notches in the bottom and only one notch on each side.

If you’re planning to add memory to a motherboard that uses SDRAM modules, you should instead upgrade the Big Three: motherboard, CPU, and memory. (Your PC is so far behind the performance of new models that it just isn’t worth adding SDRAM memory to your older motherboard. Plus, SDRAM memory is now much harder to find and is getting more expensive over time. Just chalk that up to the price of running antique hardware.)

Which Type of RAM to Buy

Here are two methods to determine which type of memory modules your current motherboard requires and which memory speeds it can handle:

  • Check the specs. Refer to the motherboard manual. Or, if you purchased your PC from a manufacturer, check the documentation that accompanied the computer. If you don’t have any manuals (think “used PC”), visit the company’s website for memory compatibility information or specifications so you don’t have to open your PC’s case until you’re ready to install the new RAM modules.

  • Check the existing modules. If you can’t find any documentation, specifications, or data on the web concerning your PC’s RAM modules, it’s time to remove the case from your computer. Look for the memory slots on your motherboard; DDR, DDR2, and DDR3 modules look like the ones shown.


    (Note: You might have more than one module already installed on your PC.) Your RAM modules might also have a descriptive label (which allows you to read the specifics without taking anything out). However, more likely, you’ll have to remove one and take it to your local computer shop.

    Remove a module; then protect the module in an empty CD-ROM jewel case when you take it for identification. The good techs, when presented with the module, should be able to tell you which memory type and speed you’re using.