While at the same speed(frequency), both DDR and DDR2 transfer the data at the same rate.
i.e the data transfer rate of DDR400 and DDR2-400 is same(3200 MBps). But DDR has the upper limit of 400MHz, which is the lower limit of DDR2- the upper limit being 1066MHz.
In case of gaming and other apps a faster memory means that the data can be transferred faster from RAM to the other components.
Like in the case of DDR2 800 MHz, the data transfer rate is 6400MBps, double of the DDR2/DDR 400MHz(3200 MBps).
A faster memory helps in faster starting/loading/reloading/exiting times in case of games.