How DDR doubles the data-rate is that when the clock edge rises, it sends data, and when the clock edge falls, it sends again. Normal memory only sends at half that rate. So 133 MHz DDR is really running effectively at 266 MHz. So that makes DDR much faster than its clock rate. This all uses pretty rough approximations.
Exact performance is far more complex.
RIMM sounds far more similar to traditional designs than they really are. RIMM's form factor won't work well in portables, and there are other issues like heat, power and shielding, so you are unlikely to see them in portables or low-end designs. Market versus Technology Just because it is better, doesn't mean it will win in the market. Heck if that was true, we'd all be using Macs. But a consortium of companies are supporting DDR for this next generation or two.