We ran a few complex test applications to verify our point.
The popular SuperPi benchmark shows quite clearly that the speed of calculating pi to 8 million decimal places depends on the memory speed. Both frequency and timings of the memory modules affect performance. On the whole, we can note that there is always a positive effect from faster DDR2 SDRAM unless its timings are the worst (highest) for that frequency.
However, not all applications are so sensitive to the memory subsystem speed. For example, PCMark05 doesn't care much about what DDR2 SDRAM you've got in your system.
The synthetic memory test from that benchmark reveals the best memory subsystem configurations, though. Its verdict differs from SuperPi's: here, we can say without any reservations that the frequency of DDR2 SDRAM has a bigger effect on performance than its latency.
The popular 3DMark06 benchmark seems to agree with the tendency, which is, however, not very sharply outlined here and is barely visible in the graphs. This serves to confirm our point that the limited bandwidth of the front-side bus is an obstacle to accelerating your system by means of high-frequency memory.