We use and recommend only name-brand memory. Commodity memory may not work properly in a given motherboard. Even if it appears to work properly at first, you may later experience subtle problems attributable to the memory. Name-brand memory costs little more than commodity memory, and is definitely worth the small extra cost.
We have installed Crucial or Kingston memory almost exclusively in the scores of systems we've built during the last several years, and have experienced no problems attributable to memory in those systems. That's something we can't say for other brands of memory we've tested, and certainly not for commodity memory. We frequently use the Crucial Memory Configurator on Crucial's web site, which allows you to enter the manufacturer and model of your system or motherboard and returns a list of memory modules, with prices, that are certified to be compatible with that system or motherboard.
Here are our recommendations for memory:
If you are upgrading a SIMM-based system, tread carefully. Such systems are now so old that major upgrades make poor economic sense. SIMM modules are quite expensive per MB, and it's quite easy to spend more on large-capacity SIMMs than the cost of a new motherboard, processor, and memory. If it's a question of adding 16 MB or 32 MB to extend the useful life of an older system, use the Crucial Memory Configurator to determine which module(s) fit your motherboard, and buy the appropriate Crucial SIMM. (http://www.crucial.com).
For maximum flexibility when upgrading an existing system, we recommend whenever possible purchasing only PC133 SDRAM memory, even for motherboards that require only PC66 or PC100. Before you do so, however, verify on the motherboard manufacturer's web site that your motherboard functions properly with PC133 SDRAM. A few motherboards designed for PC66 or PC100 memory have problems with PC133. When we need SDR-SDRAM memory, we use Crucial or Kingston modules exclusively (http://www.crucial.com or http://www.valueram.com).
If you are building a new Athlon or Pentium 4 system, we recommend using DDR-SDRAM. As is the case with SDR-SDRAM, you can nearly always use a faster module than required. For future flexibility, we recommend buying PC2700 or PC3200 DDR modules. If PC3500 modules become widely available and affordable?which we do not expect to happen?buy them instead. Buying commodity memory is always a bad idea, but when it comes to DDR, buying commodity memory is a very bad idea. DDR really pushes the limits, and using a high-quality module from a good maker is even more important than usual. If your motherboard supports both unbuffered and buffered (registered) DDR-SDRAM, consider installing registered modules for additional stability. Note that some motherboards require registered modules. We have used Crucial and Kingston DDR modules with equal success, and recommend them exclusively.
The April and May 2003 introduction of the Intel 875P- and 865-series dual-channel DDR-SDRAM chipsets effectively rendered RDRAM obsolete. If you are building a Pentium 4 system and require the highest possible memory performance, use an Intel dual-channel DDR-SDRAM motherboard and Crucial or Kingston PC3200 DIMMs.
Whatever type of memory you install, install plenty of it. For legacy Windows 9X systems, there seems to be little benefit to having more than 128 MB. For Windows NT/2000/XP, we consider 256 MB to be a good starting point, and usually install more. With the price of memory so low, we recommend you fill all of your memory slots with the largest supported modules and be done with it. We've seldom encountered a system that was having problems because it had too much memory. If you do install a large amount of memory, use ECC modules for their error-correcting capabilities.
For updated recommendations, visit: http://www.hardwareguys.com/picks/memory.html.