Proper levels of computer memory, commonly referred to as SDRAM, DDR or RDRAM (or sometimes simply as RAM or DRAM), are necessary to keep your system responding quickly. SDRAM stands for Synchronous Dynamic Random Access Memory. We recommend getting DDR (Double Data Rate SDRAM) or RDRAM (Rambus DRAM) as those types of memory are newer, faster, and will be around longer. Older model PCs and computers use only PC133 SDRAM, or even worse, PC100 SDRAM. This memory is outdated and slow by today's standards. Go for a system with DDR or RDRAM.
Go for 512 MB of RAM, no matter which type you get. If it's too much money for you, then go for 256 MB. Either way you'll probably have plenty for Windows XP or whatever new software you want on your machine. With 128 MB, Windows XP will frequently use part of your hard drive as memory swap space, slowing down your performance drastically.
RDRAM stands for "Rambus DRAM." It runs at 266, 356, 400 or 533MHz (depending on its model) and its throughput is doubled to effectively run at 532, 712, 800 or 1066MHz, and is rounded off and described in speed grades of PC600, PC700, PC800 or PC1066. RDRAM ships only with Pentium 4 systems at this time, or some outdated Pentium III systems that we don't recommend. The i820 chipset for the Pentium III that supports RDRAM did not perform well. With the Pentium 4, Intel did a much better job with the i850 chipset and RDRAM performs very well. Make sure that your system contains PC800 RDRAM or better if you are getting RDRAM. Most do, but if you have an older system you may end up with PC600 RDRAM, and you'll want to replace that. RDRAM is not used with AMD processors.
DDR stands for Double-Data Rate SDRAM. It is an updated form of "SDR" (Single Data Rate) SDRAM that supports the transfer of 2 bits of data per connection per clock cycle, as opposed to the 1 bit for standard SDRAM. If you can get DDR instead of SDRAM, do it. Just make sure that the DDR is PC2700 or PC2100. Older and rarer PC1600 DDR hardly gives you any performance increase over PC133 SDRAM. PC2700 and PC2100 DDR give you a good deal of performance increase over PC133 SDRAM, so it is worth it if you can find a system using it at a similar price. AMD Durons currently don't take full advantage of PC2100 DDR, and you most probably will not find and Intel Celeron Pentium III with DDR memory as it has a very limited system bus speed (100 MHz). Newer Pentium 4 Celerons (starting at 1.7GHz and up) have a 400MHz system bus and can take advantage of faster DDR memory, although the cheapest systems are still sold with SDRAM, limiting performance further that the Celeron already limits it. If you are getting a top of the line system you may have the option to go with PC3200 DDR. PC3200 DDR isn't widely supported yet and doesn't offer a huge advantage over PC2700. For reference, PC1600 modules use PC200/200MHz DDR, PC2100 = PC266/266MHz, PC2700=PC333/333MHz, PC3200=PC400/400MHz.
DDR vs. RDRAM vs. SDRAM
There's been a lot of talk about which memory is best. The plain truth is that when mated with a high performance system, PC2700 DDR and PC1066 RDRAM perform very similarly. If you had the choice of which to go with on a Pentium 4 system, go with the better priced RDRAM or DDR system. Don't use SDRAM with the Pentium 4 or an Athlon XP. SDRAM is older technology and should be avoided if possible in a new system as it will slow your system down.
If you want maximum SDRAM or DDR performance, check to see if CAS 2 PC133 SDRAM is used instead of CAS 3, or CAS 2 PC2700 DDR instead of CAS 2.5 PC2700 DDR. Most computer sellers don't advertise the CAS latency of their SDRAM, so it may take some effort to get the proper data from a salesperson. Even then, it will only get you a few % performance increase, so it may not be worth chasing down. RDRAM does not come in different CAS latencies.
Corporate client PC recommendations
If you mix memory types on your machines, you will face more problems if you have to upgrade all your client machines from 128 MB to 256 MB or 512 MB to support some new applications or a move to 32-bit Windows. We recommend using PC2100 DDR memory in new systems as it works across Athlon XP and Pentium 4 systems, and the future of RDRAM is uncertain. RDRAM is mainly used in some high-end Pentium 4 workstations and performance consumer systems. As for the amount of memory, go for 512 MB minimum. That will ensure that you shouldn't have to open up your computer cases to upgrade memory during the life of that computer. Time is money when it comes to IT work, and that may well be worth it if it fits into your budget. Lack of memory has been the biggest performance problem in the past. More memory means higher productivity for your clients and less calls to the help desk.