Each memory generation is designed for a certain clock speed segment. First-generation double data rate memory covered 100 MHz to 200 MHz (DDR200 to DDR400 speeds), DDR2 started at 266 MHz and maxed out at 400 MHz (DDR2-533 to DDR2-800) and DDR3 typically starts at 533 MHz and will go at least to 800 MHz (DDR3-1066 to DDR3-1600). Each DDR generation update is based on a more sophisticated production process and lower voltage and heat dissipation, which enables higher clock speeds at the expense of shorter latencies. This is why a new memory generation needs a significant clock speed lead to deliver higher performance than the preceding technology. DDR2 only outperforms DDR400 memory when running at least DDR2-667 speed, while DDR3 requires DDR3-1333 speed to show a clear benefit when compared to DDR2-800. Memories within the same generation are downwards compatible, which means that you can always purchase DDR2-1066 RAM and operate it at 667 or 800 speeds.
In addition to these industry standard speeds, there has been enthusiast overclocking memory ever since overclocking became popular in the late 90s. There were DDR DIMMs that outperformed the JEDEC standards by 50%, e.g. by DDR600 RAM by GeIL, and DDR2 reached DDR2-1200 speeds long before DDR3 was introduced. Patriot holds the record at PC10,000. The leap is even larger with DDR3, as the achievable clock speeds, in some cases up to DDR3-2000, already double the speed of the first DDR generation at 1066.
Without platforms to support such fast DDR3 memory speeds, memory beyond DDR3-1333, which is the fastest selectable speed setting with most motherboards, will serve as overclocking memory for the time being. Corsair's and OCZ's DDR3-1800 memories provide a maximum overclocking margin at this time.