Over the past few years, the cost of RAM has plummeted, making it one of the most affordable PC upgrades around. Most mainstream systems now come with 64MB RAM, and some high-end models offer 128MB. So if you're running Windows 9x on an old Pentium with 16MB and you think that your PC may be somewhat underequipped, you're right. Here's what you need to know to prepare for a successful memory upgrade.

The right type of RAM

Look to your PC's motherboard to determine the particular form of upgrade RAM you should use to produce a configuration appropriate for your system. Know your motherboard's requirements before you buy RAM, and you'll do fine. Purchase blindly, and you could prevent your system from booting, cause constant memory-error messages, or (in the worst case) even destroy the RAM itself.

Here's what you should know about the motherboard before you buy memory:

RAM type. A chip set determines the type of memory the system supports. Most first-generation Pentium chip sets support Fast Page Mode DRAM (usually called DRAM for short) or a slightly faster version known as EDO DRAM.

Some later Pentium ù and all Pentium II ù chip sets also support one of the two forms of SDRAM. Pentium and Pentium II systems running at clock speeds up to 333MHz use SDRAM designed for a 66MHz motherboard. Newer Pentium II systems running at 350MHz or faster use PC-100 SDRAM, built for a 100MHz motherboard. PC-100 SDRAM works in most older SDRAM systems, but SDRAM for 66MHz motherboards doesn't function on newer PII systems.

Socket type. Memory chips come on two types of modules: SIMMs and DIMMs. Most 386 and 486 system motherboards use 30-pin SIMM sockets, while most early Pentium motherboards are built to use 72-pin SIMM sockets. A 168-pin DIMM carries EDO DRAM or SDRAM; such DIMMs show up on later first-generation Pentium systems and on all Pentium II systems.

Though DIMMs can be installed singly, 72-pin SIMMs must be installed in pairs (called banks) of equal capacity. Some motherboards require you to fill the sockets in a specific order ù usually with the largest modules in the first banks. A few make you use modules that are all the same size.

Speed. Memory chips run at different speeds. DRAM and EDO DRAM chips typically run at 80, 70, or 60 nanoseconds (often the exterior of a DRAM or EDO DRAM chip is stamped with a number giving the chips' speed). Adding faster memory to a slower motherboard is OK but won't boost performance. Adding slower memory to a faster motherboard can cause trouble. So buy memory that's the same speed as the installed memory.

Error checking. Some RAM modules support built-in error checking (called parity) and some don't. Don't mix them. Fortunately, the majority of Pentium systems use non-parity memory, so this issue does not affect most memory buyers. You can also check the CMOS setup program; error checking has to be enabled in BIOS.

Pin coating. Old SIMMs and DIMMs have gold-coloured pins matching the colour of the motherboard's memory sockets. More recent motherboards and memory use less-expensive tin, which is silver in colour. For best results, don't mix the two metals; over a period of years an oxidation layer can build, causing memory errors.

The right amount of RAM

The amount of RAM your PC needs depends on its operating system and task load. To see how much RAM your PC currently has installed, right-click My Computer-Properties and select the General tab.

Windows 9x systems need a minimum of 16MB RAM ù and that amount is enough only if you limit your computer activity to light e-mail, word processing, Web browsing, and minimal multitasking. If you work on spreadsheets, presentations, or databases and you keep two or three applications open at a time, your system requires at least 32MB. If you multitask four or more applications and you work with graphics or play games, you need 64MB. If you run Windows NT, you require from 32MB to 128MB, depending on your task profile.

After completing the upgrade, you should notice a difference when you launch or switch between applications. Nonetheless, the more RAM you add, the less performance gain you'll see per megabyte. The law of diminishing returns rules here. Worse, some Pentium machines may actually slow down: some chip sets ù the collection of chips that control the flow of data through your PC ù provide cache support only for the first 64MB of system RAM.

The right number of modules

Besides paying attention to the characteristics of your motherboard, you'll need to determine the specific number and combination of memory modules to buy. These factors depend on the number of free sockets in your motherboard and on the specific memory module capacities that your motherboard can support.

Motherboards recognise only certain combinations of memory modules in their memory sockets. The system manual should list total system RAM amounts supported and specific memory module combinations to make up those amounts.

If your motherboard has two or more empty SIMM sockets or one or more empty DIMM sockets, you must figure out the combination of modules your motherboard needs to equal the amount of RAM that you want to add. For example, if you want to add 48MB to a motherboard that has four free 72-pin SIMM sockets, you'll probably find that the board can't accept the combination of four 12MB SIMMs or two 24MB SIMMs. In that case, you'll instead have to fill two of the vacant banks with 8MB SIMMs and the other two with 16MB SIMMs.

To add the same 48MB to a motherboard with only two free SIMM sockets, though, you'll have to take a different route. Since 24MB SIMMs are not an option, finding a SIMM pair that adds up to the 48MB needed is clearly impossible. So you'll have to settle for a SIMM pair that yields a different total capacity, or you'll have to remove two existing SIMMs and fill four sockets with some combination of SIMMs that equals 48MB plus the amount of the two removed SIMMs.

You'll need to corral all this info before you pop the top off your system and start the upgrade. If you lack the necessary documentation, call your PC manufacturer or a trusted dealer.

Now that you know what to look for when upgrading your PC's memory, turn to the Upgrader section (p156), for a step-by-step guide to installing it in your PC.