RAM chips are packaged on small, printed circuit boards that fit into slots mounted on the motherboard. These modules are usually called dual in-line memory modules (DIMMs) and single in-line memory modules (SIMMs). Most new PCs use 168-pin DIMMs. If you have an older computer (Pentium or earlier), it may need memory packaged in 72-pin SIMMs. If you have a Pentium 4 PC, it may have RIMM slots for accommodating RDRAM memory modules. Some new Pentium 4 PCs eschew RDRAM for less expensive SDRAM or DDR SDRAM, however. Synchronous Dynamic RAM (SDRAM): SDRAM has been around for about five years and includes several versions, differing primarily in bus speed: PC66, PC100, and PC133. The numbers indicate each type's speed, as expressed in megahertz. While the higher-speed memory may seem the more appealing choice, your PC chipset ultimately makes that decision. Owners of older PCs that use 66MHz DIMMs, however, can save some money by upgrading with higher-speed modules. The new memory will work, but it won't run at full speed. Double-Data-Rate SDRAM (DDR SDRAM): This is an enhanced version of SDRAM that effectively runs at double the bus speed of conventional SDRAM. Extended Data Out (EDO), Fast Page Mode (FPM): If your PC is more than five years old, it may use EDO or FPM memory. Although you can still find these memory modules--usually priced significantly higher than most newer memory types. Most likely, such a system probably needs a lot more upgrades than just memory. RDRAM, developed by Rambus and championed by Intel, is the latest high-speed memory technology. Pentium 4 PCs equipped with the Intel 850 chipset use RDRAM; newer Pentium 4 systems based on the Intel 845 chipset can use less expensive SDRAM or DDR SDRAM.