Joel Hruska, Extreme Tech, How L1 and L2 CPU caches work, and why they’re essential part of modern chips, here. Seriously, if you need someone to tell you not to touch DRAM, or L4, or L3, you should just stop in the middle of Wall Street and think about what you are doing.
This chart from Anandtech’s Haswell review is useful because it actually illustrates the performance impact of adding a huge (128MB) L4 cache as well as the conventional L1/L2/L3 structures. Each stair step represents a new level of cache. The red line is the chip with an L4 — note that for large file sizes, it’s still almost twice as fast as the other two Intel chips.