DDR3 is coming... soon. My bet is that it'll be the main system memory by middle of next year. Micron's already shipping their 1-Gbit chips to customers for evaluation. DDR2, we hardly knew ye...

"Micron will start volume production of the 1-Gbit chip early next year. It will support data rates of 800 Mbits/s to 1,600 Mbits/s and clock frequencies of 400 to 800 MHz, respectively, doubling the speed from DDR2."