The Tech Report had an indepth look at the impact of memory speed on the performance of the Intel Core i7 processor. Check it out over here.
With three channels of lowly DDR3-1066, you're looking at a whopping 25.6GB/s of memory bandwidth. Crank the memory clock up to 1600MHz, and bandwidth jumps to an even more impressive 38.4GB/s. This fat pipe, combined with the low access latencies inherent to on-die memory controllers, gives the Core i7 a formidable memory subsystem. It also poses an interesting question: how bound by memory speed is Intel's new processor architecture?
The reporter concludes that if you're cash-strapped and don't overclock, you don't lose all that much by running the slower, more affordable DIMMs in your Core i7 rig.