"DDR (DDR1) has been superseded by DDR2 SDRAM, which has some modifications to allow higher clock frequency, but operates on the same principle as DDR. Competing with DDR2 are Rambus XDR DRAM. DDR2 has become the standard, as XDR is lacking support. DDR3 SDRAM is a new standard that offers even faster performance and new features.
DDR's prefetch buffer depth is 2 bits; DDR2 uses 4 bits. Although the effective clock speeds of DDR2 are higher than for DDR, the overall performance was no greater in the early implementations, primarily due to the high latencies of the first DDR2 modules. DDR2 started to be effective by the end of 2004, as modules with lower latencies became available.
Memory manufacturers have stated that it is impractical to mass-produce DDR1 memory with effective clock rates in excess of 400 MHz. DDR2 picks up where DDR1 leaves off, and is available at clock rates of 400 MHz and higher.
RDRAM is a particularly expensive alternative to DDR SDRAM, and most manufacturers have dropped its support from their chipsets." Wikipedia http://en.wikipedia.org/wiki/DDR_SDRAM