Is DDR twice as fast as SDRAM?

In a word, Yes. DDR transfers data twice as fast as standard SDRAM modules. DDR reads data on both rising and falling edges of a clock signal. This allows DDR module to transfer data twice as fast as SDRAM. For example instead of data rate of 133MHz, DDR memory transfers data at 266 MHz.

This doesn't mean your system will run twice as fast, but your memory will. And the faster memory will improve your PC's performance.