Q&A About RAMBefore you upgrade, read this.
Stan Miastkowski is a PC World contributing editor.
Bulking up a computer's memory remains one of the most popular upgrade projects: It's inexpensive; it's relatively easy; and it can dramatically improve performance. And today's memory-hungry software can make upgrading your computer's RAM (random access memory) a virtual necessity. For example, Microsoft recommends at least 128MB of memory for Windows XP; 256MB is optimal, however, and 512MB is better still.
There are many types of RAM. Most desktop PCs built over the past few years use dual inline memory modules, or DIMMs. PC100, which runs at 100 MHz, and PC133, at 133 MHz, synchronous dynamic RAM (SDRAM) DIMMs are most common in Intel Pentium-based PCs built one to three years ago; systems of that vintage based on processors from Advanced Micro Devices often use 200-MHz PC1600 or 266-MHz PC2100 double data rate (DDR) modules.
Recently, faster DDR modules have become the standard for both Pentium- and AMD-based systems. The most prevalent type is the 333-MHz PC2700, and PCs that use 400-MHz PC3200 DDR memory are rapidly catching on. Some high-end computers use dual-channel DDR memory for extra speed. Rambus memory (RDRAM) is still available as well. High-performance Pentium 4-based systems often use more-expensive Rambus inline memory modules (RIMMs). Older machines typically have 800-MHz RIMMs, while new systems use 1066-MHz modules.
Still have questions? I've got answers.
Q: Can I mix different-capacity RAM modules?
A: This is not a problem for SDRAM (PC100 and PC133) and DDR RAM. You can fill your PC's memory slots with virtually any combination of different-capacity modules as long as they're the same type. In fact, mixing different-capacity modules is the best way to upgrade without removing the old RAM from your PC.
You can't mix SDRAM and DDR modules, though. They operate at different voltages and communicate with the system in different ways. Motherboards are designed for either one or the other.
RDRAM memory is different. You can install a single RIMM in some older motherboards based on the Intel 820 chip set. But for more-recent motherboards that use the Intel 840, 850, and 850E chip sets, RIMMs must be installed in pairs. And RIMM capacities and speeds must match.
Q: Can I mix memory modules of different speeds?
A: Yes. But all modules must be the same type. It's okay to add higher-speed modules; they just won't run at their full speed.
Q: Do I need to fill all the memory slots?
A: Not with SDRAM or DDR RAM. But with Rambus, every slot usually must be filled. You can buy continuity modules (about $10 each) that fill Rambus slots without adding memory.
If your PC is over five years old, it probably uses SIMMs, which must be installed in pairs. Most motherboards that use SIMMs have four RAM sockets, but only two need to be filled.
Q: Can I mix modules from different manufacturers?
A: Yes, as long as they're the same RAM type. Don't believe dealers who say it can't be done. They want you to buy the RAM they have available, which is usually from one manufacturer.
Q: What's ECC RAM, and should I buy it?
A: Error-Correcting-Code RAM modules have special circuitry that corrects memory errors on the fly. They're used primarily in servers and high-end workstations. ECC SDRAM will work in non-ECC motherboards, but you won't get the benefit of the ECC circuitry, so it doesn't make sense to pay the extra cost.
Q: I just filled my memory slots with 768MB of RAM. Now my system doesn't work right and locks up.
A: You're running into a problem with Windows or the limitations of your PC's BIOS. Windows 95, 98, 98 SE, and Me aren't designed to work with more than 512MB of RAM. (Windows NT, 2000, and XP don't have this limitation.) If you encounter this problem, check out Microsoft's product support.
Also, some older motherboards aren't designed to take 256MB and 512MB RAM modules. Check your manual or your PC manufacturer's Web site; sometimes a BIOS upgrade solves the problem.