from 2003 Gamers'
Hardware Upgrade Guide:
"The reason why we chose to start with a Athlon 1400 and KT266A is that our polls indicate that a large number of those who responded are running an Athlon Thunderbird or one of the "slower" Athlon XPs on a KT266A/KT333 or similar board. In fact, less than 30% of our readers are still using SDRAM-based motherboards. "
Currently  there are three main memory types still being used.
*DDR SDRAM (Double-Data-Rate SDRAM)
Example: Dell Dimension 8200 uses PC800 RDRAM [PC800 (non-ECC)].
Progression: PC800 RDRAM --> PC1066 RDRAM
Top of the line Alienware system uses 1GB PC-3200 DDR SDRAM.
Dell uses also uses PC3200 (400MHz) Dual Channel DDR SDRAM.
PC SDRAM, DDR, RDRAM
PC100 SDRAM: This type of SDRAM is designed to run up to 100 MHz speed. It's used for computers with motherboards running at 100 MHz or lower. Don't confuse motherboard speed with the processor speed of your computer. A Pentium III running 800 MHz has a motherboard running at 100 MHz. Check with your computer's manufacturer before you buy this type of RAM.
PC133 SDRAM: This type of SDRAM is designed to run up to 133 MHz speed. This SDRAM is used for computers with motherboards running at 133 MHz or less. This includes Pentium IIIs with 1 GHz and higher, Pentium IV computers, AMD Athlons, and the Apple Macintosh G4.
DDR SDRAM: Double data rate SDRAM. This type of SDRAM runs at twice the speed of standard PC100 or PC133 SDRAM by transferring data at both the beginning and end of a single computer clock cycle (For example: a 100 MHz motherboard cycle is 1/1,000.000 per second.) The only computers that can use DDR SDRAM are those based on the latest AMD Athlon (Intel Pentium-compatible) processors and motherboards.
PC1600 SDRAM: This is DDR SDRAM that doubles the transfer rate of PC100 SDRAM to 200 MHz. The reason the notation doesn't say "PC200" is because the 1600 denotes the total bandwidth the memory is capable of. In this case it's 1.6 GHz of bandwidth.
PC2100 SDRAM: This is DDR SDRAM that doubles the transfer rate of PC133 SDRAM to 266 MHz. The notation means this memory has an effective bandwidth of 2.1 GHz. Currently only the latest AMD Athlon are using PC2100 SDRAM. Intel promises DDR-compatible motherboard technology for its line of processors early next year.
RDRAM: RAMBUS DRAM is a new type of memory not based on SDRAM technology.
Developed by the Rambus company it purportedly has a faster transfer rate. However,
real-world tests have shown it is not much faster than any current RAM technology.
Currently only computers based on the Intel Pentium IV and a few Pentium IIIs