What's the deal with DDR memory?

There has been a lot of talk recently about DDR memory and how it will revolutionize our gaming systems. Coupled with the latest Athlon processors, it's been touted as the next big thing. Companies like VIA and Intel, big motherboard chipset manufacturers in their own right, are even making plans to build chipsets with support the Pentium 4. But one of the biggest problems for the potential buyer is understanding just what to buy. With all these new names and standards fogging previous conceptions, it can be mightily easy to get lost in the how and the why, and judging by the emails we have received lately, one of the biggest concerns is buying the correct sort of DDR memory. Normal SDRAM, which is what ships with practically every PC on the market these days, comes in three standards; PC66, PC100 and PC133. The number denotes the actual clock rate the memory is capable, so in this case, PC133 = 133MHz. That's the maximum front side bus the memory is rated to support. Any faster and you run the risk of instabilities and crashes. Most PCs come with PC100 SDRAM. DDR memory is different, because the standards refer not to the clock rate, which is of course the same as the SDRAM doubled by the motherboard, but the peak memory bandwidth. The two choices at the moment are PC1600 and PC2100. The 1600 in PC1600 denotes that the memory is capable of shifting 1.6Gb per second at its peak. 2100 indicates 2.1Gb/s. These are theoretical peaks, and in general use are rarely achieved. The clock rate in each case is fairly standard, 100MHz in PC1600's case, and 133MHz for PC2100. So if you intend to buy a DDR Athlon setup and the dealer tells you that it includes 128Mb of memory running at 266MHz, you know already know that the memory is PC2100 (running at 133MHz doubled by the motherboard), and that if you want any more, you should enquire as to where one can buy PC2100 DDR memory. If the dealer offers you an Athlon machine with 128Mb of 200MHz memory, because the motherboards support both speeds, you can say to him "look here, I'd like to run a 266MHz bus," and he will do the honours, no doubt passing comment about how well-informed you are on such matters. At which point you pass us a fiver and disappear into the sunset.