Depends on the processor, the processor cache, the application, and the memory speed. In the most extreme example I've tested (a s478 p4 2.8GHz Celeron) it made a real-world difference between 15 and 20 percent. The current crop of C2D chips have much more cache memory and use faster main memory, so it might be more realistic to expect a speed gain of 3-5% for dual-channel over single-channel, but that's just a guess.
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