Why will it not affect the performance? What happens to that extra 266 when it crashes into the motherboard? lol
I mean, I guess it's going to operate fine for a 1600 speed mobo, but if he bought a mobo that supported 1866 RAM, wouldn't his system run faster?
Not really. There are benchmarks that show that there's really not much difference between performance of RAM at various speeds - most of the stuff over 1600 isn't worth it even if the MOBO supports it, and I really doubt that anyone would notice any difference above 1333 in real life conditions.
Apparently Intel chips can't take advantage of faster than 1600 clock speeds anyway, though Llano chips can.
Here's an interesting article from Anandtech about RAM speed improvements versus price. Basically they conclude that there is some improvement with faster RAM, but you have to run specific benchmarks to see it and it is massively inconsistent between applications - there's a huge increase in Arkham City but less than 10% from top to bottom in Portal 2. In regular usage there would be no perceptible difference. http://www.anandtech.com/show/6372/memory-performance-1...