There are a number of things to keep in mind when you're adding RAM modules (whether DIMMs or SIMMs) to your system:

• First, make sure that the RAM modules have the same pin size and configuration as the RAM memory slots on your motherboard. For instance, older 72-pin SIMMs won't fit into 168-pin memory slots. Also make sure that the speed of the RAM is appropriate for your computer; some motherboards require faster memory than others. Also make sure that the type of RAM you get matches your motherboards requirements. Some machines are designed for a type of RAM chip called EDO (Extended Data Out) or DDR (Double Data Rate). You'll need to find new chips to match the existing chips.

• Second, adding RAM to your motherboard is actually done by filling memory "banks." Most desktop computers have at least two banks. If you're adding more than one RAM module to the same bank, make sure that both of the modules work at the same speed, which is measured in nanoseconds and megahertz. Two modules sharing the same bank can cause problems if they have different speeds.

Laptop computers typically have only one RAM expansion port. RAM designs vary widely among laptop manufacturers, so make sure you're buying the right kind of RAM for your system.

• Finally, if you're out of memory slots, you may be able to speed up your system with software that "borrows" space on your hard drive and makes your system treat it like RAM. This type of software is often called "virtual memory," and it works much slower than real RAM. Some operating system software includes virtual memory.