Advertisements usually only talk about the capacity of memory installed on systems: 1 GB, 2 GB or sometimes even 4 GB, although anything at 4 GB and above requires either a 32-bit operating system capable of handling the memory addressing to take advantage the memory capacity, or a 64-bit operating system, which natively supports much more RAM. Any system running Windows XP or Windows Vista should at least have 1 GB of memory. On the one hand, this will prevent Windows from swapping too much work data onto the slow hard drive. On the other hand, RAM has become really inexpensive, making memory one of the most affordable components.
Memory is always added to a system on so-called modules. Desktop PCs utilize Dual Inline Memory Module (DIMMs), the “dual” means that there are pins on both sides of the connector. Memory chips can also be located on both sides of a module. The first DIMMs had 168 pins and were used for SDRAM memory. The second generation for double-data rate RAM (DDR) had 184 pins, while the current DDR2 and DDR3 DIMMs have 240 pins. Differently positioned notches prevent users from confusing DDR2 with DDR3.
Due to space constraints, notebooks and laptops utilize different memory modules, which are called Small Outline DIMMs (SO-DIMMs). These are roughly 1/3 of the size of regular DIMMs, and have only 200 pins. Most notebooks offer two sockets for SO-DIMMs, although ultra-portables oftentimes come with permanently installed memory and only one slot for upgrades.
It’s important to know that the best performance is achieved by pairing two physical memory banks, which are referred to as channels. All memory controllers, whether they’re integrated into an AMD Athlon 64 X2 or Phenom X3/X4 processor or part of a chipset northbridge, are capable of running dual-channel mode, which means that they utilize two banks in order to double the bandwidth by widening the memory data path from 64 bits to 128 bits.
Good RAM, Bad RAM
While memory is categorized by memory type (DDR2, DDR3) and memory speed (clock speed), there are also various parameters that need to be set, called timings, and they’re also relevant to performance. However, the performance difference between mainstream memory and high-end RAM has been rather small. It is highly important to use a reasonable memory technology, which currently is DDR2-667/800 for notebooks and DDR2-800/1066 for desktop PCs. Running fast memory timings is favorable, but we recommend against spending a lot of money on enthusiast memory unless you’ve already optimized other key components such as the processor or the graphics card. In the case of notebook memory it’s usually not possible to alter memory timings anyway.
Mix or Match?
Memory vendors try to sell so-called matched memory pairs, which are often tested together in a standardized test environment. While two different DIMMs with identical specification should technically work together, issues may occur. Huge compatibility problems, resulting in functional incompatibility, are very rare these days, but it’s fairly possible that RAM speed or timings may have to be adjusted to slower settings for compatibility reasons.
You can always operate any memory type at slower clock speeds and more conservative timings than they were specified for. Running a DIMM above the specification, though, puts you into the overclocking corner, and must be done at your own risk. If you mix a 1 GB DDR2-667 and a 1-GB DDR2-800 DIMM, both will run at DDR2-667 speed.