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Memory Speed and the System Cache
It is important to remember that the use of large, efficient primary and secondary caches in modern PCs means that most requests for instructions or data by the processor are satisfied from cache and not the system memory. While increasing the speed of memory definitely has a positive performance impact, often that improvement is masked by the fact that typically, only 5-10% of all memory requests actually result in memory accesses. This means that using memory that is 50% faster than an older kind will only result in a system-level performance gain of 2.5-5%. This phenomenon is the main reason why SDRAM, despite being much faster than its predecessors, has not revolutionized the PC, only evolved it.
See this section in the discussion on cache to see how the system memory fits into the "big picture" of the PC.