The market and the players
It would be logical to assume that DDR2 is the successor to DDR, but if that is the case it is taking a while for the transition to occur.
DDR2 has been around for over 12 months on graphics cards, but the base standard and architecture differs from that used on a motherboard module. On motherbards, DDR2 is only supported by Intel based systems, with AMD expected to launch compatible models sometime during 2006, having decided to stick with DDR for their current generation of products.
Faster data rates, increased frequencies, expected support by both major CPU manufacturers and a more power efficient module, all these factors point to increased use by desktop PC owners of DDR2, and with latencies falling all the time DDR2 looks increasingly certain to be the future of memory design, but DDR will still be around for a while.
When thinking about DDR and DDR2 RAM you need to distinguish between the chip makers and the module makers. Various module makers will take the chips of a particular chip maker and use them in their products. You'll find different module designs and testing result in differing performance RAM modules all coming from the same chips. Some cheaper RAM modules are often 'speed binned' chips that didn't pass tests at higher speeds.
Companies you'll hear when talking about chips include Hynix, Samsung, Micron and Winbond. Manufacturers of memory modules include Corsair, Crucial, OCZ and Mushkin, although there are many more – too many to mention here.
If you've heard that a certain chip model overclocks well, it's important to remember that you cannot solely rely on the type of chip being used as an indicator of how the RAM will perform; it depends on the entire module.