There is barely any performance difference between the systems working with two and three memory channels in applications that are not too sensitive to the timings changes.
However, different number of memory channels may still tell on the gaming performance in some titles. For instance, FarCry 2 reveals 2% performance drop when we switch from three to two memory channels. Nevertheless, this is a truly ridiculous difference, especially against the background of the 33% drop of the theoretical peak bandwidth.
In fact, the effect from losing one of the memory channels can only compare with setting less aggressive memory timings. For example, the results of our tests show that dual-channel DDR3-1600 is in most cases faster than triple-channel DDR3-1200 SDRAM.
According to the results of MaxxPi2 benchmark, dual-channel DDR3-1600 SDRAM with 8-8-8-24 timings can provide almost the same level of practical performance as triple-channel DDR3-1600 with 9-9-9-24 timings.
So, using dual-channel memory in Core i7 systems instead of triple-channel DDR3 SDRAM recommended by Intel is a totally acceptable solution that allows to save some money and at the same time lose not too much of the performance. That is why we strongly recommend checking out dual-channel memory kits instead of triple-channel ones made of 1GB modules. However, you should remember about one thing. Most high-speed dual-channel 4GB kits are initially designed for LGA775 systems and require higher voltage settings. DIMM voltage increase is a painful question for LGA1366 platforms, so when you will be choosing a dual-channel memory kit for a Core i7 system, keep an eye on its ability to work without any serious voltage increase.